Emit audit event for agent creation and deletion

This change set emits two new audit events: one when a new cluster agent
is created and one when a cluster agent is deleted or attempts of the
two.

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/462749

Changelog: added
Issue: https://gitlab.com/gitlab-org/gitlab/-/issues/462749
EE: true
MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/159593