[Feature] Set priorityClassName on kiam-server and kiam-agent Pods

[daviddyball]

In almost all cases I usually want kiam pods to be some of the very first to come up with a node (with the exception of any K8s control plane or CNI pods).... so perhaps we can set priorityClassName on the manifests provided in the repo?

Not sure if this is worthy of being the default, but to me it makes sense to have spec.template.priorityClassName: system-cluster-critical.

Thoughts? I'm happy to raise a PR if people agree.