Add support for Storage Classes and pre-provisioned PVs

[ConaghFM]

Add support for Storage Classes and pre-provisioned PVs

This PR adds support for two features for the Netbird Management pod PVC:

• Specifying a storage class
• Specifying an existing Persistent Volume

Storage Class

Users may wish to use a non-default Storage Class.

• Added value .Values.management.persistentVolume.storageClass to populate the .spec.storageClassName field in the PVC manifest
• The implementation preserves the underlying behaviour of the storageClassName field, i.e. empty string ("") has subtly different behaviour to no value (null).
• The implementation also preserves the previous default behaviour of the chart - setting no value for the storageClassName field, thus the change is non-breaking

Pre-existing Persistent Volumes

Some users may wish to provision volumes separately from the chart (i.e. static provisioning), for example when migrating configuration data between environments or storage types.

• Added value .Values.management.persistentVolume.existingPVName to populate the .spec.volumeName field in the PVC manifest