Audit Events for Merge Request Approver Modifications
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### Audit need
When Merge Request approvers are modified (added or removed), there is no way to determine who made these changes or when the modifications occurred.
### Proposal
Whenever Merge Request approvers are changed, create an Audit event capturing:
- The user who modified the approver list
- The action performed (approver added or removed)
- The identity of the approver that was added or removed
- The timestamp of the change
### Streaming-only event or normal event?
Normal event (to support both stdout/logs and audit events storage)
###Technical Details
Event type: `merge_request_approver_added` or `merge_request_approver_removed`
issue