Add gitlab-ci to build and publish protobuf python client library

Description

Add gitlab-ci, matching process for OpenAPI client build as much as possible, to build and publish client library for protobuf messaging.

Further details

Prototype CI exists at: https://gitlab.developers.cam.ac.uk/uis/devops/experiments/mk2155/protobuf-prototype

Should run library build if:

• buf.yaml file exists
• Standard template CI "not disabled"

Should publish to repository registry on tags/releases.

Should attach built wheel/tar to release as asset.

Task list

Acceptance criteria

• CI exists to build and publish client libraries.

Links/references