[Feature] Fleet: Update run should support notification and manual trigger after stage

Kubernetes Fleet Manager strategies allow the user to pause after a stage for a certain period of time. In this way, an update to the next stage is rolled out on a time-based manner and may not be checked/tested to see if everything is OK after the update.

It would be very helpful to send a notification to an action group as soon as the status of the stage run has changed, so that the state of the cluster could be checked/tested and then the next stage could be triggered manually.

An alternative would be if a notification could at least be sent after a stage so that a stage update is not missed.