Allow tracker publisher to post to the GitLab MRs

Current situation

Tracker publisher can actually post the quality gate result to the GitLab issues only. At the same time, it uses the gitlab plugin in tools that allows for posting the results to the issues and to the merge requests. It is thus simple and interesting to extend the tracker publisher posting capabilities.

Desired outcome

If the workflow contains gitlab/mr label, the tracker publisher posts the quality gate results as note to the specified gitlab merge request.