WIP: Cache Merge Requests highlighted diffs

What does this MR do?

Cache highlighted diffs for merge requests when they are requested for existing merge requests but after each change on the merge request diff recalculate again the cache.

What are the relevant issue numbers?

Closes #20034 (closed)

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together