Filter merge requests approved by user (backend)

What does this MR do?

Backend work to allow filtering merge requests approved by specified users

The following new search filters were added to a Merge Request:

• approved_by_usernames
• approved_by_ids

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

Part of #39042 (closed) (backend)