Create a system note when an alert is escalated
When an alert comes in via an alert integration, users will be notified of the alert following the escalation policy for the project.
Each time a (batch of) notifications is sent out, a system note should be logged on the alert. The note should list who was notified and which escalation policy was being followed.
For additional context, see https://gitlab.com/gitlab-org/monitor/monitor/-/issues/56#note_510409735.