Rust code generated from protobufs should be in source control

[Jake-Shadle]

When doing cross-compilation just now I had to install make because of protobuf-src, which reminded me that this project is building protobuf from source as well as generating the Rust code during build. This is extremely expensive in build time, particularly for CI. It would be much nicer to commit the generated Rust code to remove the entirety of the build time cost. They would only need to be regenerated when updating tonic/prost or the protobufs themselves. This can be done via a tool like https://github.com/EmbarkStudios/proto-gen that can also validate the generated code is up to date if needed.

[Jake-Shadle]

This was resolved in #896