✨ Mount vm snapshot disk

[cc005511]

What does this PR do, and why is it needed?
VirtualMachineSnapshotDiskSpec is used to define a specific disk from a VirtualMachineSnapshot. VirtualMachineSnapshotDiskSpec field added in VirtualMachineVolumeSource is to support mounting a VirtualMachine snapshot disk as a volume of the VM service VM.

• Only one member (virtualMachineSnapshot or persistentVolumeClaim) of VirtualMachineVolumeSource could be specified at a time. The webhook validation will reject a VirtualMachineVolume whose source sets both persistentVolumeClaim and virtualMachineSnapshot.
• If virtualMachineSnapshot is set in VirtualMachineVolumeSource, the requester must have the permission to get, create, or delete VM snapshot in the VM snapshot namespace.
• The name or the diskID of VirtualMachineSnapshotDiskSpec must not be empty.
• If virtualMachineSnapshot is set in VirtualMachineVolumeSource but the VirtualMachineSnapshot is not-found or not-ready, the specific error will be set in status.volumes[].error.
• Once the volume is attached (VirtualMachine.status.volumes[].attached = true), changes on the volume source (the name or the diskID) is not allowed.

PASS
go test -v ./pkg/providers/vsphere/session/...
go test -v ./webhooks/virtualmachine/validation/...
go test -v ./controllers/virtualmachine/volume/...

Are there any special notes for your reviewer:
Expose the disk list in VirtualMachineSnapshot: #1744
CRD change of supporting VM snapshot disk as the volume source: #1751

Please add a release note if necessary:
Support mounting a VM snapshot disk as a VM volume. When using the VM Snapshot disk as the volume source, diskMode must be set to "IndependentNonPersistent" and removable must be set to true.