Add "controllerutil" helpers for applyconfigurations (SSA)

[erikgb]

We are making extensive use of the helpers defined in https://github.com/kubernetes-sigs/controller-runtime/blob/main/pkg/controller/controllerutil/controllerutil.go, and while migrating our controllers to SSA we had to create our own modified copies based on corev1/metav1 applyconfigurations.

I think it could make sense adding a SSA-flavoured edition of these simple (but useful) helpers to controller-runtime. And I would be happy to submit a PR if you agree. The most challenging is probably to decide the package name/location. WDYT @alvaroaleman @vincepri @sbueringer?

[troy0820]

/kind support feature

[troy0820]

The tricky part here is the applyconfigurations that are not available for custom resources. (Should we deal with controller-tools to have that be created when you scaffold kubebuilder?)

Do we only plan to support this for corev1 and the like? I think server side apply was looking to be done on the client.Client interface as well. Doing this in controllerutils may help migrate that to the interface. I'm interested in this as well.

[erikgb]

I just want to add the same helper as we have for the normal types. Here is an example from the modified helper we are using now:

import (
	"fmt"

	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
	"k8s.io/apimachinery/pkg/runtime"
	metav1ac "k8s.io/client-go/applyconfigurations/meta/v1"
	"k8s.io/utils/ptr"
	"sigs.k8s.io/controller-runtime/pkg/client/apiutil"
)

// SetControllerReference sets owner as a Controller OwnerReference on controlled.
// This is used for garbage collection of the controlled object and for
// reconciling the owner object on changes to controlled (with a Watch + EnqueueRequestForOwner).
func SetControllerReference(owner metav1.Object, controlled *metav1ac.ObjectMetaApplyConfiguration, scheme *runtime.Scheme) error {
	// Validate the owner.
	ro, ok := owner.(runtime.Object)
	if !ok {
		return fmt.Errorf("%T is not a runtime.Object, cannot call SetControllerReference", owner)
	}
	if err := validateOwner(owner, controlled); err != nil {
		return err
	}

	gvk, err := apiutil.GVKForObject(ro, scheme)
	if err != nil {
		return err
	}
	controlled.WithOwnerReferences(
		metav1ac.OwnerReference().
			WithAPIVersion(gvk.GroupVersion().String()).
			WithKind(gvk.Kind).
			WithName(owner.GetName()).
			WithUID(owner.GetUID()).
			WithBlockOwnerDeletion(true).
			WithController(true),
	)
	return nil
}

[erikgb]

This might be possible to solve with generics also. I can look into a generics solution, as it will probably make the code more compact with less duplication.