[EKS] [addons/CoreDNS]: Expose priorityClassName for CoreDNS as EKS addon

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**Tell us about your request**

Expose `PriorityClassName` as part of CoreDNS addon schema.

To avoid failures on CoreDNS pods we're running them on on-demand instances, but if we would be able to switch pods' priority class to something that Karpenter will not recognize as critical, we would be able to migrate CoreDNS on spots.

**Which service(s) is this request for?**
EKS, CoreDNS addon

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**

I'm trying to maintain EKS cluster on spot EC2 instances, where these nodes are provisioned by Karpenter.
Unfortunately Karpenter has unorthodox order of draining pods (where critical pods are drained last, see: https://github.com/kubernetes-sigs/karpenter/issues/2217), which combined with spot instances termination period may cause critical pod to not be cleanly drain.

**Are you currently working around this issue?**

To avoid failures on CoreDNS pods we're running them on on-demand instances, but if we would be able to switch pods' priority class to something that Karpenter will not recognize as critical, we would be able to migrate CoreDNS on spots. (we're also contemplate moving to self-managed CoreDNS).

The issue names the EKS CoreDNS addon and its addon schema; start by locating that schema and the configuration path for CoreDNS pod priority. Confirm how PriorityClassName is exposed and verify that CoreDNS pods receive the requested priority class without disrupting addon behavior.

Written by the indexing model from the issue text.