Feature Request: 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.

[karlkfi]

ClusterSelectors should work in OSS. You just have to configure the cluster name on the reconciler-manager Deployment or the ConfigMap with the same name, using the CLUSTER_NAME environment variable or --cluster-name option, instead of having it read from a custom resource populated by the GKE Fleet service. See the e2e tests for an example: https://github.com/GoogleContainerTools/kpt-config-sync/blob/main/e2e/testcases/cluster_selectors_test.go

When using Config Sync with GKE Fleets, the Fleet service writes a Membership custom resource to each cluster with the name. The Config Sync reconciler-manager reads this resource automatically, to pick up the membership name as the cluster name.

I'm not sure what you mean by ClusterRegistry though. Do you mean GKE Fleets or something else?