Support pyi_out in py_proto_library

[David Sanderson]

For issue https://github.com/protocolbuffers/protobuf/issues/10366 (Support pyi_out in py_proto_library) I have proposed https://github.com/protocolbuffers/protobuf/pull/21567 (Generate .pyi files in py_proto_library).  I have left this as a draft because I do not yet have a passing CI run.  However, in order for CI to run, I apparently need a maintainer to apply a tag to my PR.

If I could please get a maintainer to apply the 'safe for tests' tag to my PR, I'd appreciate it.

Thanks,

David Sanderson