Replace aiopykube with kr8s in controller

[jacobtomlinson]

Taking a more methodical approach to #696 and replacing one thing at a time.

This first PR adds kr8s as a dependency, defines the custom resource objects in the controller and updates handle_scheduler_service_status() to use kr8s for setting the DaskCluster status.

[jacobtomlinson]

All of the failures here appear to be the same as in #670