The GRPC channel used in KesselCheckClient might be compromised or closed for different reasons, including a Kessel server restart or a network issue. Notifications needs to be able to recover from such an issue.

Acceptance criteria:

• GRPC channel issues can be detected automatically (use the channel state).
• All GRPC calls are performed with retries. Exceptions while calling Kessel trigger a retry.
• The retry policy is configured from the ClowdApp template.
• An alert based on a new metric is fired if the connection with Kessel cannot be (re-)established.