Implement a minimal, internal, experimental rust_proto_library rule.

The internal design is consistent with other <lang>_proto_library rules. rust_proto_library attaches rust_proto_library_aspect on its `deps` attribute. The aspect traverses the dependency, and when it visits proto_library (detected by ProtoInfo provider) it registers 2 actions:

1) to run protoc with Rust backend to emit gencode
2) to compile the gencode using Rustc

Action (2) gets the Rust proto runtime as an input as well.

Coming in a followup is support and test coverage for proto_library.deps.

PiperOrigin-RevId: 514521285