Store merge request ref_fetched status in the database

What does this MR do?

It introduces ref_fetched database field for merge request, so we can cache the value and avoid hitting git when displaying a merge request.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

🛫 🚆

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [ ] Documentation created/updated
• [ ] API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #34052 (closed)