Add ci protobuf generator

Closes #3 (closed)

Most work was done by @mk2155 in https://gitlab.developers.cam.ac.uk/uis/devops/experiments/mk2155/protobuf-prototype. I made a couple of changes to match what we have in main currently. I also skipped the code logic to automatically create __init__.py files, as this is not needed anymore now we make use of betterproto.