Audit event when agent is created and deleted

We want to be able to track agent deletion actions. Thus, we should create an audit event whenever an agent is deleted: via UI, REST API and GraphQL API.

Release notes

As you work on maintaining the auditability of your systems it's critical to know when a component that can access your system gets added or removed. As the agent for Kubernetes allows bi-directional data flow between a Kubernetes cluster and GitLab, knowing about new or removed agents is a central piece of auditability. Until now compliance teams had to use custom tooling or search for this data in GitLab directly. GitLab now provides the following audit events:

  - `cluster_agent_created` records who registered a new agent for Kubernetes
  - `cluster_agent_create_failed` records who tried to register a new agent for Kubernetes but failed
  - `cluster_agent_deleted` records who removed an agent for Kubernetes registration
  - `cluster_agent_delete_failed` records who tried to remove an agent for Kubernetes registration but failed

These audit events extend the already existing cluster_agent_token_created and cluster_agent_token_revoked audit events to improve the auditability of your GitLab instance.