Add separate PVC for tiered storage cache

[eduardagarici]

Add tiered storage cache as a separate PVC

Summary

• Introduces a tieredStorageCache: true flag on StorageConfig that marks a volume as a Kafka tiered storage cache (e.g. rsm.config.fetch.chunk.cache.path). Cache volumes are provisioned as independent PVCs, separate from
  Kafka log volumes.
• Cache volumes are excluded from log.dirs so Kafka does not treat them as data directories.
• Cache volumes are excluded from Cruise Control capacity calculations and disk rebalance operations — CC must not account for ephemeral storage.
• Controller-only nodes skip cache PVCs entirely since they do not serve broker traffic.
• Shrink resize: because cache data is ephemeral, koperator supports shrinking a cache PVC via a safe delete-and-recreate flow coordinated with the rolling upgrade machinery. The old PVC is annotated pending-deletion, a
  replacement is created immediately (provisioning overlaps with gate evaluation), and the old PVC is deleted once the broker pod stops. A grow continues to use the standard Kubernetes in-place expansion path. Full details in
  docs/tiered-storage-pvc-resize.md.

Test plan

• testTieredStorageCachePvcResize passes against a local kind cluster — verifies the full shrink lifecycle (staging, pod cycle, annotation cleanup, correct final size)
• Cache volume does not appear in log.dirs on the broker
• CC capacity config does not include the cache mount path
• Normal log volume grow/shrink behaviour is unaffected