dynamic host volumes: enforce exclusive access in plan apply

[tgross]

Some dynamic host volumes are claimed by allocations with the capability we borrowed from CSI called single-node-single-writer, which says only one allocation can use the volume, and it can use it in read/write mode. We enforce this in the scheduler, but if evaluations for different jobs were to be processed concurrently by the scheduler, it's possible to get plans that would fail to enforce this requirement. Add a check in the plan applier to ensure that non-terminal allocations have exclusive access when requested.

Ref: https://hashicorp.atlassian.net/browse/NET-11549

[gulducat]

I got a mental hiccup seeing CSIVolume...(HostVolume...) -- I'd think it should be

Suggested change

	CSIVolumeAccessMode(HostVolumeAccessModeSingleNodeSingleWriter) {
	HostVolumeAccessMode(HostVolumeAccessModeSingleNodeSingleWriter) {

[tgross]

Unfortunately not (ref volumes.go#L100-L101) because the type for VolumeRequest is named for the CSI mode.

// VolumeRequest is a representation of a storage volume that a TaskGroup wishes
// to use.
type VolumeRequest struct {
	Name           string
	Type           string
	Source         string
	ReadOnly       bool
	Sticky         bool
	AccessMode     CSIVolumeAccessMode
	AttachmentMode CSIVolumeAttachmentMode
	MountOptions   *CSIMountOptions
	PerAlloc       bool
}

But both are "defined types" on string, which msgpack serializes to the same thing on the wire. So we could swap out for a new shared VolumeAccessMode / VolumeAttachmentMode without breaking backcompat. Especially because in this case we didn't define types in the api package version of these fields. They're just strings there!

I'll do that, but under a separate PR.

[tgross]

#24896