The zuul-config repo now defines a gitlab-pipeline job that shows how we
can delegate control of the CI workflow back to GitLab from Zuul so that
projects can self-serve their own CI in the canonical GitLab way while
still benefiting from Zuul functions such as speculative repo state and
gating.

The job is started on zuul-executor and requires no nodepool, making for
minimal startup latency. It interacts solely with the GitLab API to:

 1. Resolve MR details such as the source branch.
 2. Create a CI pipeline for the source branch. The pipeline becomes
    associated with the MR in GitLab automatically and displays in the
    UI just as any normal MR triggered pipeline.
 3. Passes the Zuul Build UUID to the pipeline so that it can retrieve
    speculative state source files (with dependencies) over rsync
    instead of cloning from the GitLab repo.
 4. Waits on the GitLab pipeline to complete.
 5. Fails or succeeds according to the pipeline status.

See `repos/test-project/.gitlab-ci.yml` for the project config needed to
handle Zuul triggered pipelines.

Bug: T350288