[RUN AS-IF-FOSS] Add basic reviewers quick actions

What does this MR do?

This is the first part of an implementation of #241244 (closed). It adds the /assign_reviewer and /unassign_reviewer commands based heavily on the /assign and /unassign commands.

I'm making a few assumptions based on the code- please correct me if these are wrong!

1. Reviewers are not a part of Issues - only Merge Requests
2. Multiple Reviewers will not be a core feature just as Multiple Assignees wasn't
3. These commands are still behind a feature flag (merge_request_reviewers)

The /reassign_reviewr command will be introduced in !47606 (merged)

Screenshots (strongly suggested)

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

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Related to #241244 (closed)