py: Add CI job to publish pyln-grpc-proto package to PyPI

[cdecker]

We have most of our python packages under control of https://github.com/ElementsProject/lightning/blob/master/.github/workflows/pypi.yml

However pyln-grpc-proto is different, in that it doesn't quite adhere to the structure that the other pyln packages have, hence why it was removed from that deployment script.

We need to re-add it.