Move gitlab build to CI-template

Description

This issue is to move the written gitlab-CI code over to the ci-template repository so that it can be reused.

Further details

Task list

• Copy existing code to sensible location in ci-template (matching OpenAPI behaviour)
• Ensure compatibility with other common-pipeline files (e.g. multi-stage-docker)
• Use ci-template version in this repository instead of local files

Acceptance criteria

• Client libraries built using ci-template code rather than local build.
• protobuf:publish:python:gitlab to run on default branch if no rules override is done (see #3 (comment 807062))
• Documentation available how to use this template
  • Should include information about the requirement of using the release-it merge request flow (see #3 (comment 806774))

Links/references