ci: Add manual job to trigger QA

It's currently more complicated than necessary to trigger a QA run with changes which are currently in review. As a result, we tend to just not bother and wait for QA to crash at a later stage, when changes had been merged to the master branch already and when GitLab is about to bump the Gitaly server version. While we certainly don't want all MRs to be tested by QA, it would be reasonable to do so for MRs which e.g. deprecate existing features or which implement significant changes like for example the recent Go ports.

Improve the situation by adding a new manual "qa" job. If executed, then it triggers a QA pipeline with the tip of the MR as Gitaly server version.