Closes #3081.

This adds support in the Gazelle plugin for controlling how the generated py_proto_library rules are named; support for these was originally added in #3057. We do this via a new Gazelle directive, python_proto_naming_convention, which is similar to python_library_naming_convention and the like, except it interpolates $proto_name$, which is the proto_library rule minus any trailing _proto. We default to $proto_name$_py_pb2.

For instance, for a proto_library named foo_proto, the default value would generate foo_py_pb2, aligning with the convention stated in the Bazel docs.