As a software engineer, I want to ensure the podplacementconfigs.multiarch.openshift.io resource is a singleton, and users can create one and only one object (for example named `cluster`) for that resource.

Acceptance criteria:

• creating an object with name different than "cluster" or more than one PodPlacementConfig object is denied.

In kube 1.28, ValidatingAdmissionPolicy is set as beta, and we could use them instead of a validating admission webhook, but this needs evaluation as it could prevent GAing until that feature is GA too.

https://kubernetes.io/docs/reference/access-authn-authz/validating-admission-policy/