Support --incompatible_enable_proto_toolchain_resolution

[kusaeva]

Is your feature request related to a problem?

The problem is need to compile protoc instead of using prebuilt binary while building java_grpc_library

Describe the solution you'd like

Support --incompatible_enable_proto_toolchain_resolution bazel flag so users can override protoc compiled from sources with prebuilt binary, as already done in rules_python or WIP in rules_go

Describe alternatives you've considered

Additional context

Examples of declarations toolchains for other languages that already support the feature here

[kannanjgithub]

Feel free to raise a PR for this.

[mortenmj]

@kannanjgithub I had a look at what it would take to make a PR for this, and there are two parts.

One is using a toolchain to provide protoc, to avoid the dependency on @com_google_protobuf//:protoc_lib. That is pretty straight forward. I could make a separate PR for that, as it would be pretty small and easy to review.

The second part involves the dependency on @com_google_protobuf//:protoc_lib, which I don't think is very easy to avoid. If we make it easy for the user to declare their own java_rpc_toolchain, users could opt to consume prebuilt binaries from maven while still maintaining the existing default of building from source. Does that sound like something to move ahead with?