Expose default attr 'compiler' about bazel-gazelle

[jayconrod]

At the moment go_grpc_library is just a wrapper for go_proto_library with a specific compiler. I'd like to make Gazelle migrate existing instances of go_grpc_library to go_proto_library (see #8).

I think it's reasonable to have an option that explicitly sets the compiler attribute on generated go_proto_library rules though.

FYI, you can use buildozer to make bulk changes like this to rules. Should be easier than forking and modifying Gazelle.

[johnskopis]

After a little bit more thought it probably makes more sense to allow caller to override the implementation of go_proto_library or go_rpc_library instead

[Globegitter]

It would be nice if this could also be defined for the go_repository, so e.g. I could specify

go_repository(
        name = ".....",
        commit = "....",
        importpath = "...",
        build_file_proto_compiler = "@io_bazel_rules_go//proto:gogofaster_grpc",
    )

But I suppose that is more specific to the rules_go repo. So opened an issue there for it.