Draft: Have StuckCiJobsWorker execute DropPendingWorker

What does this MR do?

In !63314 (comment 605193249) @reprazent suggested:

As far as the StuckCiJobsWorker worker is concerned. If we've hit the ceiling optimizing these queries, I wonder if we could make sure that a failure of 1 of the drop operations doesn't prevent any of the others from running. Perhaps by breaking up the worker? Or is the sequence these are executed important?

This extracts out a Ci::StuckBuilds::DropPendingWorker and Service.

This is pulled off of this Large MR (to be used as reference for what's coming next).

!64643 (closed)

Screenshots (strongly suggested)

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