grpc: crate maintenance

[dfawley]

1. Rework the codegen crate:

• protoc-gen-rust-grpc is now a real Rust crate that is able to build protoc (for now) and our plugin binary.
• grpc-protobuf-build uses protoc-gen-rust-grpc to produce the binaries, instead of requiring the user to put them in their path.
• Reworked the caching of the binaries a little. I thought we'd be able to use the standard Rust caching, but it didn't work for various reasons. So maybe this isn't much better than what was there before. LMK.
• We now run cargo build for interop inside the CI's environment instead of in the run environment which uses MSYS and for some reason doesn't like the way we're doing the caching. (Maybe this is a good enough reason to go back to how the cache was working before?)

2. Add a grpc/README.md, and add more info to Cargo.tomls, remove publish=false to allow them to be published, etc.