CSI volume mount option updates while volume in use

Hey there,

Currently Nomad [doesn't allow updating](https://github.com/hashicorp/nomad/blob/27b1d470a8f0c8c39a3afb72ca12c425a35cf376/nomad/structs/csi.go#L822) a volume registration's mount options while the volume is in use. Is there a specific reason for this?

Ideally I'd like to be able to update mount options on a volume registration and restart the alloc to pick up those changes. This is especially important when using Nomad's Terraform provider to manage volumes.

Start with nomad/structs/csi.go at the linked volume-registration validation around line 822, then trace how registration changes and allocation restarts handle mount options. Done means an in-use volume registration can accept updated mount options and a restarted allocation uses them, with the relevant behavior covered by tests.

Written by the indexing model from the issue text.