Allow merge request diffs to be placed into an object store

What does this MR do?

Allows the content of merge request diffs to be stored in an external object store, rather than postgresql

Checklist:

• Fix MergeRequestDiffFile to work with GitlabUploader (need a primary key)
• Configure a diffs object storage endpoint
• Select between database and object storage for diffs
• Make collection lookups efficient

What are the relevant issue numbers?

Closes #52568 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• [-] Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer