Add Rust Bzlmod support for #299

[PSmouz]

Summary

Closes #299 for Rust by adding Bzlmod Rust support with the standard language-module rule surface:

• rust_proto_compile
• rust_grpc_compile
• rust_proto_library
• rust_grpc_library

What Changed

• Adds modules/rust with:
  • Rust proto/gRPC plugin declarations
  • Rust compile/library rule generation support
  • rules_rust + crate-universe based dependencies
  • generated Rust crate fixup support for multi-plugin output
• Adds generated Rust docs and README entries.
• Adds standalone Bzlmod examples for all four Rust rules.
• Adds Rust routeguide client/server examples and wires Rust into the routeguide matrix.
• Adds Rust example CI tasks for Linux, macOS, and macOS arm64.
• Adds out-of-the-box handling for common proto types:
  • google.protobuf.* through prost_types
  • google.type.* / google.rpc.* through proto_types
  • example coverage for Timestamp, Duration, and Money

[aaliddell]

Thanks! Sorry I haven't had time to review this yet; between work and others I haven't had the time to dedicate this repo recently