CreateDockerWorkerVersion should allow adding refs to a revision of an existing version

Let's say you push a new tag on the master branch of a worker. GitLab will run two CI pipelines, one for the commit on master, and one for the same commit on your new tag. In the future, both pipelines will be running arkindex workers publish to publish the new worker. Only one of the two commands will be successful though, because they will both call CreateDockerWorkerVersion with the same arguments, except for revision_refs which will be set to one branch ref in one pipeline, and one tag ref in the other. When CreateDockerWorkerVersion finds that a worker version already exists on a revision, it fails:

{"__all__": ["A worker version already exists on this revision and this worker slug."]}

To support pushing on multiple branches or to support tags, we need to have CreateDockerWorkerVersion support creating a new GitRef on a revision of an existing worker version. We could also introduce a new CreateGitRef endpoint, but the command would have to list every single worker version, find the one matching its arguments, take its revision ID and create the ref itself, which is a lot of work that the backend can do much more easily.