The GetPodsUsingPVC function had O(N*M) complexity - for each PVC,
it listed ALL pods in the namespace and iterated through each pod.
With many PVCs and pods, this caused significant performance
degradation (2+ seconds per PV in some cases).
This change introduces a PVC-to-Pod cache that is built once per
backup and reused for all PVC lookups, reducing complexity from
O(N*M) to O(N+M).
Changes:
- Add PVCPodCache struct with thread-safe caching in podvolume pkg
- Add NewVolumeHelperImplWithCache constructor for cache support
- Build cache before backup item processing in backup.go
- Add comprehensive unit tests for cache functionality
- Graceful fallback to direct lookups if cache fails
Fixes#9179
Signed-off-by: Shubham Pampattiwar <spampatt@redhat.com>