API support for MR merge to temporary merge ref path

What does this MR do?

Add a merge_requests/:iid/merge_to_ref API which make use of the groundwork to write merge results into refs/merge-requests/:iid/merge.

What are the relevant issue numbers?

closes https://gitlab.com/gitlab-org/gitlab-ce/issues/57252

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer