**COMMIT PER COMMIT!**

_That's the nasty one... there's always a nasty one 😬_ 

Introduces the `DataLoader` trait, which can load any file, either from
a filepath by reading from the local filesystem, or from a pre-loaded
file's contents.

`DataLoader`s are run in parallel where possible, and have complete say
over what data they are interested in or not (i.e. they are not
registered based on an extension, which is very limited in practice).

I've decided to commit small example assets for the types we support,
cause I feel we really need a fast way of checking whether everything
still works from time to time.
It's pretty light but might be controversial, I figure.

Checks:
- [x] `cargo r -p rerun-cli --no-default-features --features
native_viewer -- examples/assets/example.{glb,gltf,obj,jpg,png,rrd}`
- [x] Native: `File > Open > examples/assets/*`
- [x] Native: `Drag-n-drop > examples/assets/*`
- [x] Web: `File > Open > examples/assets/*`
- [x] Web: `Drag-n-drop > examples/assets/*`

---

Part of a series of PRs to make it possible to load _any_ file from the
local filesystem, by any means, on web and native:
- #4516
- #4517 
- #4518 
- #4519 
- #4520 
- #4521 
- TODO: register custom loaders
- TODO: high level docs and guides for everything related to loading
files

What the title says.


![image](https://github.com/rerun-io/rerun/assets/2910679/68f2e499-f4df-4e75-95f6-0ed7f479c5e6)


Checks:
- [x] `cargo r -p rerun-cli --no-default-features --features
native_viewer --
examples/assets/example.{glb,gltf,obj,jpg,png,rrd,txt,md}`
- [x] Native: `File > Open > examples/assets/*`
- [x] Native: `Drag-n-drop > examples/assets/*`
- [x] Web: `File > Open > examples/assets/*`
- [x] Web: `Drag-n-drop > examples/assets/*`

---

Part of a series of PRs to make it possible to load _any_ file from the
local filesystem, by any means, on web and native:
- #4516
- #4517 
- #4518 
- #4519 
- #4520 
- #4521 
- TODO: register custom loaders
- TODO: high level docs and guides for everything related to loading
files

`.ply` support, but point clouds only, at least for now.

No feature flag: I had some, and tbh it only made thing needlessly hard
to work with for very little gains.
`.ply` are so common anyway, they always come up in user requests.

Here's an example from
[there](https://github.com/HuangCongQing/Point-Clouds-Visualization/blob/master/2open3D/data/fragment.ply):

![image](https://github.com/rerun-io/rerun/assets/2910679/8242c5a2-037d-46c6-9816-ab9b240a7473)


Checks:
- [x] `cargo r -p rerun-cli --no-default-features --features
native_viewer --
examples/assets/example.{glb,gltf,obj,jpg,png,rrd,txt,md,ply}`
- [x] Native: `File > Open > examples/assets/*`
- [x] Native: `Drag-n-drop > examples/assets/*`
- [x] Web: `File > Open > examples/assets/*`
- [x] Web: `Drag-n-drop > examples/assets/*`

---

Part of a series of PRs to make it possible to load _any_ file from the
local filesystem, by any means, on web and native:
- #4516
- #4517 
- #4518 
- #4519 
- #4520 
- #4521 
- TODO: register custom loaders
- TODO: high level docs and guides for everything related to loading
files

Load folders filled with whatever. Go nuts.

The loader has no particular semantics (e.g. HIVE partitioning): it just
loads everything in a folder using the appropriate loader(s) for each
file.

Checks:
- [x] `cargo r -p rerun-cli --no-default-features --features
native_viewer -- examples/assets`
- [x] Native: `File > Open > examples/assets/`
- [x] Native: `Drag-n-drop > examples/assets/`

---

Part of a series of PRs to make it possible to load _any_ file from the
local filesystem, by any means, on web and native:
- #4516
- #4517 
- #4518 
- #4519 
- #4520 
- #4521 
- TODO: register custom loaders
- TODO: high level docs and guides for everything related to loading
files

…#4521)

Turn any executable in your `PATH` into a `DataLoader` by logging data
to stdout.

Includes examples for all supported languages.


![image](https://github.com/rerun-io/rerun/assets/2910679/82391961-9ac1-4bd8-bdbe-a7378ed7a3fa)


Checks:
- [x] `rerun-loader-rust-file` just prints help message
- [x] `rerun-loader-rust-file
examples/rust/external_data_loader/src/main.rs` successfully does
nothing
- [x] `rerun-loader-rust-file
examples/rust/external_data_loader/src/main.rs | rerun -` works
- [x] `rerun-loader-python-file` just prints help message
- [x] `rerun-loader-python-file
examples/python/external_data_loader/main.py` successfully does nothing
- [x] `rerun-loader-python-file
examples/python/external_data_loader/main.py | rerun -` works
- [x] `rerun-loader-cpp-file` just prints help message
- [x] `rerun-loader-cpp-file examples/cpp/external_data_loader/main.cpp`
successfully does nothing
- [x] `rerun-loader-cpp-file examples/cpp/external_data_loader/main.cpp
| rerun -` works
- [x] `cargo r -p rerun-cli --no-default-features --features
native_viewer -- examples/assets`
- [x] Native: `File > Open > examples/assets/`
- [x] Native: `Drag-n-drop > examples/assets/`


---

Part of a series of PRs to make it possible to load _any_ file from the
local filesystem, by any means, on web and native:
- #4516
- #4517 
- #4518 
- #4519 
- #4520 
- #4521 
- TODO: register custom loaders
- TODO: high level docs and guides for everything related to loading
files

Introduce a first-class protocol for `DataLoader`s -- whether they are
builtin, custom, or external -- to announce that they don't support
loading a given piece of data.

This fixes one of the oldest issue when loading files in Rerun: loading
unsupported data would always bottom out in either the image or
websocket paths, leading to unrelated errors very confusing to users.

The loading process is now a two-pass process:
1. Dispatch the data to be loaded to all loaders, which will respond
ASAP whether they can do it or not.
1.1. If at least one compatible loader is found, go to 2
1.2. Otherwise, fail early with a nice error message for the end user
2. Dispatch the actual data loading.

This has important/subtle ramifications on the threading model, but
other than that is what you'd expect.


Checks:
- [x] Native: CLI examples/assets/*
- [x] Native: CLI examples/assets
- [x] Native: CLI examples/assets/* containing unsupported files
- [x] Native: CLI examples/assets containing unsupported files
- [x] Native: File>Open examples/assets/*
- [x] Native: File>Open examples/assets
- [x] Native: File>Open examples/assets/* containing unsupported files
- [x] Native: File>Open examples/assets containing unsupported files
- [x] Native: Drag-n-drop examples/assets/*
- [x] Native: Drag-n-drop examples/assets
- [x] Native: Drag-n-drop examples/assets/* containing unsupported files
- [x] Native: Drag-n-drop examples/assets containing unsupported files
- [x] Web: File>Open examples/assets/*
- [x] Web: Drag-n-drop examples/assets/*
- [x] Web: File>Open examples/assets/* containing unsupported files
- [x] Web: Drag-n-drop examples/assets/* containing unsupported files



---

Part of a series of PRs to make it possible to load _any_ file from the
local filesystem, by any means, on web and native:
- #4516
- #4517 
- #4518 
- #4519 
- #4520 
- #4521 
- #4565
- #4566
- #4567

Add support for registering custom loaders, the same way we offer
support for registering custom space views and store subscribers.

Checks:
- [x] cargo r -p custom_data_loader -- Cargo.toml

---

Part of a series of PRs to make it possible to load _any_ file from the
local filesystem, by any means, on web and native:
- #4516
- #4517 
- #4518 
- #4519 
- #4520 
- #4521 
- #4565
- #4566
- #4567

Some docs for the whole thing.

It heavily relies on linking to the examples for exact details and to
avoid rot.

It's probably extremely bad. Help welcome.


---

Part of a series of PRs to make it possible to load _any_ file from the
local filesystem, by any means, on web and native:
- #4516
- #4517 
- #4518 
- #4519 
- #4520 
- #4521 
- #4565
- #4566
- #4567

Co-authored-by: Nikolaus West <niko@rerun.io>