Enable garbage collection of custom resources

Enhance the garbage collector to periodically refresh the resources it
monitors (via discovery) to enable custom resource definition GC.

This implementation caches Unstructured structs for any kinds not
covered by a shared informer. The existing meta-only codec only supports
compiled types; an improved codec which supports arbitrary types could
be introduced to optimize caching to store only metadata for all
non-informer types.

pkg/controller/garbagecollector/errors.go

@@ -20,8 +20,6 @@ import (
 	"fmt"
 )
 
-const nonCoreMessage = `If %s is a non-core resource (e.g. thirdparty resource, custom resource from aggregated apiserver), please note that the garbage collector doesn't support non-core resources yet. Once they are supported, object with ownerReferences referring non-existing non-core objects will be deleted by the garbage collector.`
-
 type restMappingError struct {
 	kind    string
 	version string
@@ -36,7 +34,6 @@ func (r *restMappingError) Error() string {
 func (r *restMappingError) Message() string {
 	versionKind := fmt.Sprintf("%s/%s", r.version, r.kind)
 	errMsg := fmt.Sprintf("unable to get REST mapping for %s. ", versionKind)
-	errMsg += fmt.Sprintf(nonCoreMessage, versionKind)
 	errMsg += fmt.Sprintf(" If %s is an invalid resource, then you should manually remove ownerReferences that refer %s objects.", versionKind, versionKind)
 	return errMsg
 }