Spike: Provide a way to retrieve a list of open MRs that change a specific file

Spike for: #448868

Context

To assess viability of the feature proposal When visiting a file (blob) page provide a way ... (#448868) we'd like to gain an understanding of the complexities of building an endpoint/query (GraphQL) that retrieves the open MRs that include changes to a given file.

Details/Open questions

• Does it matter the branch where the blob is being viewed?
  • ie, when visiting a file file1.txt in a branch feature-123 any MR touching the path file1.txt (adding, removing, renaming, deleting) should be listed? Or only MRs touching that path AND have as target branch feature-123?
• What performance considerations should be taken into account?
  • Should the list be capped/paginated right away?
• ... (please add more) ...