Add audit events for merge request settings

What does this MR do and why?

Capture audit events when users modify merge request settings via app and API

• Merge method
• Merge options
• Squash commits when merging
• Merge checks
• Merge suggestions

Screenshots or screen recordings

How to set up and validate locally

1. Run GDK
2. Go to Projects Settings -> General and Expand Merge requests.
3. Change the settings and check for audit events in Project audit events page, Group audit events page and admin/audit_logs

Related to #301124 (closed)