Draft: Add MergeRequests::TriggersMergeStatusSubscription concern

What does this MR do and why?

This makes the #trigger_merge_request_merge_status_updated re-usable in services not extending the MergeRequests::BaseService class.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #372525 (closed)