Log user instance request and rejection in Audit Events

Proposal

As part of &4491 and &4315, we should track when a member requests to join an instance, when their request to join an instance is approved, and when their request to join an instance has been rejected. There are already Audit Event logs for user request approval: #276250 (closed). This issue introduces Audit Event logs for instance access request and request rejection.

Relates to: #276250 (closed) which logs an audit event on user registration approval. Also relates to: #286611 (closed), which logs application logs on user approval/rejection.