Implement StreamApprovalAuditEvent worker/service

What does this MR do and why?

This is part of the effort to reduce the SQL queries executed by the MergeRequests::ApprovalService. This moves streaming approval audit event to another subscriber to MergeRequests::ApprovedEvent.

This is behind async_after_approval feature flag.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #10869 (closed)