nfs: Add support for nfs shallow volumes

[Rakshith-R]

Describe the feature you'd like to have

Add support for nfs shallow volumes based on cephfs shallow volumes
A nfs shallow volume will need to use a different path(/.snap/) to mount volumes on node.

• https://github.com/ceph/ceph-csi/blob/devel/docs/design/proposals/cephfs-snapshot-shallow-ro-vol.md#node-operations
• https://github.com/ceph/ceph-csi/blob/devel/internal/nfs/controller/volume.go#L150C24-L150C24

What is the value to the end user? (why is it a priority?)

User will benefit from the same use cases as cephfs https://github.com/ceph/ceph-csi/blob/devel/docs/design/proposals/cephfs-snapshot-shallow-ro-vol.md#use-cases for nfs.

How will we know we have a good solution? (acceptance criteria)

nfs shallow volumes should work similar to cephfs shallow volumes.

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[Rakshith-R]

Currently we cannot export a path other than cephfs filesystem root or subvolume.

Therefore, we cannot expose the snaps in .snap directory.
This issue maybe resolved by ceph/ceph#53925

Moving this to next release.