Add mergeability check batch worker
What does this MR do and why?
This adds mergeability check worker that processes the list of given merge requests in order to avoid triggering too many mergeability check workers at once. It just adds the worker for now as the first step and this will be used in the subsequent MR.
MR acceptance checklist
This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.
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I have evaluated the MR acceptance checklist for this MR.
Related to #410125