feat: add directory backed UserVolumes

[shanduur]

New field in UserVolumeConfig - volumeType that defaults to partition, but can be set to directory.
When set to directory, provisioning and filesystem operations are skipped and a directory is created under /var/mnt/<name>.

The directory type enables lightweight storage volumes backed by a host directory, instead of requiring a full block device partition.

When volumeType = "directory":

• A directory is created at /var/mnt/<metadata.name>;
• provisioning, filesystem and encryption are prohibited.

Note: this mode does not provide filesystem-level isolation and inherits the EPHEMERAL partition capacity limits.
It should not be used for workloads requiring predictable storage quotas.

apiVersion: v1alpha1
kind: UserVolumeConfig
name: test
volumeType: directory

Resolves #11848

[shanduur]

We pass /var/mnt to kubelet as read-only, need some different handling of mount here - e.g. with bind?