Cache list group MR response

What does this MR do and why?

This MR introduces caching to the List group MRs endpoint. It does this behind an FF to control what groups would be eligible. It also adjusts the cache TTL to 30 minutes. Both modifications are done to prevent the effects of a stale cache from being widespread and likely.

References

Spike: identify opportunities to improve top-le... (#585222 - closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

• enable the FF for a group via rails console
• tail app logs tail -f log/application_json.log | jq.
• call the endpoint /api/v4/groups/:group_id/merge_requests
• logs should show List group merge requests endpoint caching allowed

MR acceptance checklist

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