Implement kluster deletion

[databus23]

This needs a general decision how we want to delete the Kluster:

1. React to the TPR deletion event. That would be here
2. Set the TPR status to something like Deleting and only delete the TPR once everything is removed from k8s and other backend systems.

I favour approach number 2 because it maintains visibility during cluster deletion (same like it is with pods). If we do that we should at least also thing about garbage collecting clusters that have their TPR manually deleted.