Add _registered_method to grpc ChannelStub

gRPC team is working on a change (https://github.com/grpc/grpc/pull/35850) which includes adding a new parameter to generated stubs.

This change will break some tests because this parameter doesn't exist in [ChannelStub](https://github.com/googleapis/python-api-core/blob/main/google/api_core/grpc_helpers.py#L498).

We need to add this new _registered_method parameter to ChannelStub.