Fire a `deployment_approved`/`deployment_rejected` audit event when deployment is approved/rejected

Proposal

Extend the continuous delivery audit events with deployment_approved/deployment_rejected. The even should include the representedAs attribute when set.

Implementation

It looks like the process for adding audit events is well documented here: https://docs.gitlab.com/ee/development/audit_event_guide/

The entry point for these audit events is likely https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/app/services/deployments/approval_service.rb for approval and rejection. There's no separate rejection service class so we'll want to make sure the code stays readable for handling the conditions or create a new issue to refactor.