[Improvement] Add priorityClassName support to Gravitino Helm charts

### What would you like to be improved?

The Gravitino, Iceberg REST, and Lance REST Helm charts do not expose `priorityClassName` on the pod spec. On shared clusters where batch jobs compete with production services, Kubernetes uses PriorityClass to decide which pods to evict. Without this, Gravitino pods have no priority tier and can be evicted by lower-priority workloads during resource pressure, causing unexpected downtime for production catalog services.

### How should we improve?

Add `priorityClassName: ""` to all three charts' `values.yaml` and conditionally render it in the pod spec in each `deployment.yaml`:

Applies to:
dev/charts/gravitino
dev/charts/gravitino-iceberg-rest-server
dev/charts/gravitino-lance-rest-server

Start with values.yaml and deployment.yaml in dev/charts/gravitino, dev/charts/gravitino-iceberg-rest-server, and dev/charts/gravitino-lance-rest-server. Compare the pod-spec sections and existing optional fields, then render the charts to verify that priorityClassName is omitted when empty and present when configured. Done means all three charts expose and conditionally render the value.

Written by the indexing model from the issue text.