Add support for ClusterRegistry and ClusterSelectors

[CmdrSharp]

ClusterRegistry & ClusterSelectors are a powerful part of Anthos Config Management that make multi-cluster repositories easier to manage. I would propose that this functionality (or similar functionality) gets implemented as a native part of kpt-config-sync. At the moment, the required CRD's are being applied; but can not be utilised to selectively target configuration.