feat: Add access mode metric for PersistentVolumes

[viragvoros]

What this PR does / why we need it:
This PR adds a kube_persistentvolume_access_mode metric that exposes the access modes of PersistentVolume objects, in a way that mirrors the existing PVC access mode metric.

At the moment, if we want to filter or aggregate PersistentVolumes by access mode (e.g. ReadWriteOnce, ReadWriteMany, ReadOnlyMany), we have to rely on kube_persistentvolumeclaim_access_mode and join through
PVC metrics. That forces us to scrape the PVC access mode metrics even when we only care about PVs, and increases metric cardinality.

By exposing a dedicated PV access mode metric with a small label set, we can:

• filter and aggregate PVs directly by access mode
• avoid scraping PVC access mode metrics in setups where they are not
  otherwise needed,
  while still keeping metric cardinality low.

Details of the change

• Add kube_persistentvolume_access_mode metric in the PV store:
  • creates one sample for each access mode a PersistentVolume lists
  • access_mode label uses the standard Kubernetes access
    modes (ReadWriteOnce, ReadOnlyMany, ReadWriteMany, etc.)
  • metric follows a similar style as the existing PVC access mode metric
• Update internal/store/persistentvolume_test.go
  • add test coverage for the new PV access mode metric
• Update PV metrics documentation

How does this change affect the cardinality of KSM:
Previously, dashboards and alerts that needed PV access mode information had to join against PVC access mode metrics and therefore scrape kube_persistentvolumeclaim_access_mode, which may have higher cardinality in large clusters.
With kube_persistentvolume_access_mode, users can query PV access modes directly from the PV metrics.

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