Protobuf rules using Tonic

[tvolk131]

I'm looking into porting one of my Rust projects over to Bazel. It uses Protobuf/gRPC, however it is built on Tonic rather than stepancheg's Protobuf/gRPC libraries which are used in the existing proto rules. I've used both and I personally much prefer Tonic, as it generates cleaner and simpler Protobuf objects, the gRPC stubs use async/await, and the whole library is an integrated part of the Tokio stack. It'd be really nice to have separate rules for Tonic. Something like rust_tonic_library.

[dfreese]

I put up an very WIP attempt to integrate what my team uses into rules_rust here: #479

I, unfortunately, haven't had the time to knock out the details on that.

[UebelAndre]

There's another (updated) PR for #595

[simonfong6]

I'm trying to port a Tonic/Tokio project from crates to Bazel as well