Introduce singularNameProvider for core types

This introduces `singularNameProvider`. This provider will be used
by core types to have their singular names are defined in discovery
endpoint. Thanks to that, core resources singular name always have
higher precedence than CRDs shortcuts or singular names.

pkg/registry/core/namespace/storage/storage.go

@@ -291,6 +291,13 @@ func (r *REST) ShortNames() []string {
 	return []string{"ns"}
 }
 
+var _ rest.SingularNameProvider = &REST{}
+
+// SingularName implements the SingularNameProvider interfaces. This returns singular name of core resource.
+func (r *REST) SingularName() string {
+	return "namespace"
+}
+
 var _ rest.StorageVersionProvider = &REST{}
 
 func (r *REST) StorageVersion() runtime.GroupVersioner {

pkg/registry/core/namespace/storage/storage_test.go

@@ -628,3 +628,11 @@ func TestShortNames(t *testing.T) {
 	expected := []string{"ns"}
 	registrytest.AssertShortNames(t, storage, expected)
 }
+
+func TestSingularName(t *testing.T) {
+	storage, server := newStorage(t)
+	defer server.Terminate(t)
+	defer storage.store.DestroyFunc()
+	expected := "namespace"
+	registrytest.AssertSingularName(t, storage, expected)
+}