feat: add kube_persistentvolume_volume_mode metric

[ricardoapl]

What this PR does / why we need it:

Add volumeMode of PersistentVolumes as a separate new metric of its own

How does this change affect the cardinality of KSM: (increases, decreases or does not change cardinality)

Increases (by adding a new metric)

Which issue(s) this PR fixes:

Fixes #1966

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[logicalhan]

/triage accepted
/assign @mrueg

[mrueg]

/lgtm

Thanks for your contribution!

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: mrueg, ricardoapl

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