[image_picker] Add desktop support

[stuartmorgan]

(This is the combination PR for overall review.)

This adds initial desktop support across all three desktop platforms:

• Adds the concept of a camera delegation as discussed in [image_picker_windows] The Windows implementation of the image_picker should support ImageSource.camera. flutter#102115, and adds a helper base class at the platform interface level for implementations that want to use that approach. (Sharing it simplifies both the platform implementations and the client usage.)
• Adds a new app-facing and platform-interface API to check at runtime for support for a given source, to make writing code using image_picker in a world of dynamic camera support viable. (In particular this is critical for published packages that use image_picker, as they won't know in advance if a client app will have a delegate set up).
• Updates the Windows implementation to:
  • use the new delegation base class
  • implement the newer getImageFromSource API as an opportunistic fix while changing the code (if not implemented, it would fall back to getImage)
  • use new APIs in its example.
• Made macOS and Linux implementations that are clones of the Windows approach. They are both slightly simpler however, as MIME type (for Linux) and UTI (for macOS) allow creating generic "image" and "video" filters instead of having to hard-code a file extension list.

Since in my opinion this level of support is sufficient to allow basic real-world use of the desktop implementations, due to having a structure in place for handling camera support, this includes endorsing the desktop implementations. It is still the case that desktop does not support image resizing, which isn't ideal, but I don't think we should block on that. It is clearly documented in the README.

The desktop implementations are separate packages even though they are mostly the same right now because:

• the amount of (non-example) code being duplicated is small, and
• it makes it much easier for us to change platform implementations over time; e.g., recent versions of macOS support PHPicker, meaning we can later add native code for that to the macOS implementation, with just a fallback to the current file_selector wrapper. It's plausible that better native options for Windows and/or Linux may become available in the future as well.

Fixes flutter/flutter#102115
Fixes flutter/flutter#102320
Fixes flutter/flutter#85100

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter. (Unlike the flutter/flutter repo, the flutter/packages repo does use dart format.)
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I listed at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is exempt from version changes.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

[stuartmorgan]

As usual for adding new platforms, this looks like a ton more code to review than it actually is; most of it by volume is example boilerplate:

• The Dart code in the implementation package examples is copied directly from the (slightly modified in this PR) Windows code
• All of the non-Dart code in the examples is just flutter create boilerplate.

[stuartmorgan]

I did this in a single PR because I wanted to approach it from the standpoint of "what do we need to get to something end-to-end workable to endorse desktop. It was useful for discovering things like, it works much better for everyone if the camera delegation goes into the platform interface as an option, rather than duplicated in each platform.

Now that I have that though, I'm happy to carve it up into a series of PRs if you would prefer smaller reviews:

• Update the platform interface with the delegation concept.
• Update Windows to use delegation.
• Add an unendorsed macOS implementation.
• Add an unendorsed Linux implementation.
• Add the new app-facing API and endorsement.

[stuartmorgan]

Overriding changelog: The remaining macOS changes in this PR are just the binary files from the example app that I accidentally didn't transfer to the other branch because I created it using a patch file.