Generate types to deserialize simple extensions

[mbrobbel]

Adds Rust type generation based on the YAML schema for simple extensions using typify in a new text module. This allows typed (de)serialization of extensions files.

In addition this applies the following refactoring:

• Renamed the protobuf module to proto to better reflect the structure in the substrait main repo
• Add jobs to test the different features (also added them to the bors config)
• Updated the REAMDE with some additonal links and instructions (replaces Add doc links to README #34)
• Remove Cargo.lock file (libraries should not check in their lock file https://doc.rust-lang.org/cargo/faq.html#why-do-binaries-have-cargolock-in-version-control-but-not-libraries)
• Some updates to the Cargo.toml file (added features, include license and core extensions in package, set msrv and set homepage to substrait.io)
• Split different codegen steps in build.rs to different functions
• Add crate documentation about serde features
• Add protoc feature to build protoc from source in build step

This is blocked on substrait-io/substrait#387 and substrait-io/substrait#404 (to allow the generated code to deserialize the extension_types.yaml file). Deserialization of the extension files from the substrait submodule are tested in https://github.com/substrait-io/substrait-rs/pull/27/files#diff-f0c7b6d48022c11a573ef12664de310c63ab020e21ee0c442cb7340975663466R14.

Note that this is a breaking change.

[andygrove]

LGTM