Rewrite protoc-gen-connect-python in Python

We don't *have* to do this, but it seems ideal that we'd implement the plugin in Python to ease contributions to the repo, consolidate on tooling and testing, etc. (Admittedly, Go _is_ pretty nice for writing plugins.)

If we do choose to do this, we could take a look at https://github.com/fischor/protogen-python — the "original" connect-python's plugin was written with this library.