Add creates a new Seed-Sync controller and sets up Watches.

CleanupFinalizer is put on Seed CRs to facilitate proper cleanup when a Seed is deleted.

This controller is responsible for synchronizing the `Seed` custom resources across all seed clusters.

ManagedByLabel is the label used to identify the resources created by this controller.

Reconciler copies seed CRs into their respective clusters, assuming that Kubermatic and the seed CRD have already been installed.