bug: CachedObjects are populated with PartialObjectMetadata

[gman0]

Describe the bug

When the replication for CachedResources writes the objects to cache, they only contain PartialObjectMetadata instead of full objects.

Steps To Reproduce

1. Create a ConfigMap and a CachedResource for it:

   apiVersion: v1
   kind: ConfigMap
   metadata:
     name: hello-green
     labels:
         color: green
   data:
     hello: world
   ---
   apiVersion: cache.kcp.io/v1alpha1
   kind: CachedResource
   metadata:
     name: hello-cache-green
   spec:
     group: ""
     version: v1
     resource: configmaps
     labelSelector:
       matchLabels:
         color: green

2. Read the object from the cache with a cache client, observe it contains metadata.

Expected Behaviour

The cached objects should be replicated fully.

Additional Context

This is happening because the replication controller for CachedResources gets its data from DiscoveringDynamicSharedInformerFactory which is instantiated with a metadata client.

kcp/pkg/server/server.go

Lines 155 to 175 in 802765f

	metadataClusterClient, err := metadataclient.NewDynamicMetadataClusterClientForConfig(
	rest.AddUserAgent(rest.CopyConfig(s.MiniAggregator.GenericAPIServer.LoopbackClientConfig), "kcp-partial-metadata-informers"))
	if err != nil {
	return nil, err
	}
	crdGVRSource, err := informer.NewCRDGVRSource(s.ApiExtensionsSharedInformerFactory.Apiextensions().V1().CustomResourceDefinitions().Informer())
	if err != nil {
	return nil, err
	}
	s.DiscoveringDynamicSharedInformerFactory, err = informer.NewDiscoveringDynamicSharedInformerFactory(
	metadataClusterClient,
	func(obj interface{}) bool { return true },
	nil,
	crdGVRSource,
	cache.Indexers{},
	)
	if err != nil {
	return nil, err
	}

Normally, this would be okay because DiscoveringDynamicSharedInformerFactory is heavy and metadata is usually sufficient for where this factory is being used at the moment. In context of replication this is however not enough and we need full objects, with a real dynamic client that doesn't strip objects.