WIP: Add _links to MergeRequest API Entity

What does this MR do?

It adds a _links attribute to the MergeRequest entity that holds links to: self, ssh, closes_issues, target_project, source_project, target_branch, and source_branch.

Please let me know if some of these are not worth adding, or if there are other related resources that would be worth having in this list.

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

Is a test that checks that the required keys are there enough, or should I test against expected values?

Why was this MR needed?

See #19078 (closed)

Screenshots (if relevant)

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

What are the relevant issue numbers?

Closes #19078 (closed)