Expose `priorityClassName` and `preemptionPolicy` in chart values for Alpha and Zero pods

[gautambhat]

The current Dgraph Helm chart doesn’t allow setting a priorityClassName for the Alpha and Zero StatefulSets.

This makes it hard to integrate with clusters that use pod priorities for scheduling (e.g., database pods that must preempt lower-priority workloads).

Proposed change:
Add optional values like:

priorityClassName: ""
preemptionPolicy: ""

And wire them into the alpha and zero templates.

This matches conventions in many official Helm charts and would make it easier for downstream operators to override priority settings in GitOps flows.