Cache `#can_be_resolved_in_ui?` git operations

What does this MR do?

Cache #can_be_resolved_in_ui? git operations

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

No

Why was this MR needed?

MergeRequests::Conflicts::ListService fetches the full conflict diffs on each call, which translates into a large volume of git operations. Especially in the post-gitaly world, this is unsustainable.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

Closes gitaly#1051 (closed)