Build basic mergeability framework

Based on the exploration and design discussions in #300042 (closed), build the basic framework that can report mergeability status such that it returns a default false response (assuming our failure mode is to block MRs, rather than allow them)

Open/Unscoped Questions

• Which API/endpoints does this status appear in?
• Should this be behind a feature flag, or can we build this as a parallel Source of Truth?

To Do

• sync with @pedroms on UX implications

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