Add Rust Tonic and Prost support

[aaliddell]

Either as additional rules or to replace the current rules as breaking change. These are better supported than the current grpcio etc crates and avoid the flakeyness of the current grpc-sys workarounds.

To do this requires that they have protoc plugins, which neither currently have since they share the same builder core. Currently blocked on:

• https://github.com/tokio-rs/prost/issues/330 / https://github.com/tokio-rs/prost/issues/463 / https://github.com/tokio-rs/prost/pull/492
• https://github.com/hyperium/tonic/issues/750

[titanous]

I'm interested in implementing this. I think it would make sense to remove the existing rules and replace them with new rules (that have tonic and prost in the rule names in case other libraries become popular in the future).

[aaliddell]

Do the upstream libraries actually have working protoc plugins yet? There seem to be some external ones, but are these official?

[titanous]

They aren't official but they seem to work fine.

[UebelAndre]

https://github.com/bazelbuild/rules_rust/issues/915#issuecomment-1079742714 may be useful here.

[titanous]

I've made good progress implementing this, but I've run into an issue where the tonic (gRPC) plugin wants to write to the output files from the prost plugin, and I can't see an existing way to make this happen in compile.bzl. Suggestions?

[titanous]

I've implemented this in #202.

[aaliddell]

Closed by #265