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.

[madelinevibes]

Bounty Info in CLN's discord

[ThomsenDrake]

PR #8999 addresses this issue — it adds pyln-grpc-proto to the PyPI publishing workflow with a custom Makefile target that handles the directory layout and versioning differences.

Happy to adjust the approach if the maintainers have preferences on how the CI matrix or version sync should work.