Technical Investigation: Merge request approval rule for code coverage

Summary

To resolve #15765 (closed) we have decided to implement a new merge request approvals rule similar to License and Vulnerability checking.

We need to research how to implement the frontend and backend for this feature.

Questions to answer

frontend

• How do multiple approval rules stack?
• How do we create the project setting?
• How do we determine the state of the merge request merge component?
• Is there anywhere else we need to change in order to create a merge request approval?

backend

• How will we determine when the coverage has increased/decreased against the target branch?
• How do we create the merge request approval rule?
• Is there anything else we need to do in order to hook up the coverage rule to the merge request page?
• What do we do if coverage is unavailable on the source branch? Target branch?