`objc_proto_library` bazel rule

[jschear]

What language does this apply to?
Objective-C.

Describe the problem you are tryfing to solve.
I have some .proto files, I'd like to consume them in a Bazel build.

Describe the solution you'd like
There's a great cc_proto_library in //Bazel:bazel_cc_proto_library.bzl, but there's no objective-C equivalent. It looks like there was once a version built in to Bazel, but it was removed.

Describe alternatives you've considered
I'm currently using rules_proto_grpc's rule, but it would be nicer not to have to pull in additional dependencies.