Merge request sha info

What does this MR do?

Exposes sha and merge_commit_sha items in MergeRequest API endpoint data.

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

No.

Why was this MR needed?

It's useful information.

What are the relevant issue numbers?

Closes #20456 (closed).

Screenshots (if relevant)

N/A

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