Add symmetric filter helped based on OwnerRefable. (#2032)

Most resources stamped out by knative controllers have OwnerReferences synthesized via `kmeta.NewOwnerRef`, which requires the parent resource to implement `kmeta.OwnerRefable` for accessing the `GroupVersionKind`.

However, where we setup informer watches on those same children resources we have essentially relied on direct synthesis of the `Group[Version]Kind` where we could instead provide an empty instance of the controller resource and leverage `GetGroupVersionKind` to provide the GVK used for filtration.

So where before folks would write:

```golang
FilterFunc: controller.FilterControllerGK(v1alpha1.WithKind("MyType"))
```

They may now write:

```golang
FilterFunc: controller.FilterController(&v1alpha1.MyType{})
```

The latter is preferable in part because it is more strongly typed, but also shorter.

controller/controller.go

@@ -152,6 +152,13 @@ func FilterControllerGK(gk schema.GroupKind) func(obj interface{}) bool {
 	}
 }
 
+// FilterController makes it simple to create FilterFunc's for use with
+// cache.FilteringResourceEventHandler that filter based on the
+// controlling resource.
+func FilterController(r kmeta.OwnerRefable) func(obj interface{}) bool {
+	return FilterControllerGK(r.GetGroupVersionKind().GroupKind())
+}
+
 // FilterWithName makes it simple to create FilterFunc's for use with
 // cache.FilteringResourceEventHandler that filter based on a name.
 func FilterWithName(name string) func(obj interface{}) bool {