feat: support WAL PVC storage class selection

[dkarter]

Motivation

PgDog's WAL PVC currently inherits the cluster's default StorageClass. That can produce a disk the PgDog node pool cannot attach; for example, a pd-balanced volume on a C4A node that requires Hyperdisk.

Summary of Changes

• Add optional statefulSet.walPvc.storageClassName to the StatefulSet claim template.
• Leave it unset by default so existing installs keep their current behavior. An explicit empty string disables dynamic provisioning, as in Kubernetes PVCs.
• Add tests for omitted, named, and empty storage classes. Bump the chart to v0.83.

Testing

./test/test.sh passes. To check the output, render with test/values-statefulset-storage-class.yaml and confirm the wal volume claim template has storageClassName: balanced-storage. Render with test/values-statefulset.yaml and confirm it has no storageClassName field.

Dependencies/Special Considerations

This only affects newly created claims. Existing WAL PVCs keep their assigned StorageClass; check for outstanding prepared transactions before considering any PVC replacement.

[levkk]

Same comment as last PR, let's stop adding tests here.

[dkarter]

oh my bad i missed the comment on the last pr!