Feature: allow user to specify the `priorityClassName` for `PolicyServer`

[flavio]

Starting from Kubewarden 1.25, the PolicyServer CRD has been extended with an optional attribute named spec.priorityClassName. This is a string that holds the name of a Kubernetes priority class to be used when scheduling the Policy Server workload.

This is entirely optional, when not specified the value is either nil or empty.

Acceptance criteria

• Update the UI dialog used by user to create custom PolicyServer resources. The user should be able to enter a priority class name if he wants to. It would be nice to have a drop down menu that shows all the priority classes that are available, this would avoid mistakes from the user.
• (Optional) when the Kubewarden helm chart is deployed through the UI, and the user provided a custom value for the priorityClassName, then when creating a new PolicyServer the UI should show the priorityClassName used by the helm chart. The user will then decide if he wants to keep the same priorityClassName or remove it or use a different one.

[cnotv]

The resource to be fetched is scheduling.k8s.io.priorityclass