refactor: volume request modes to be generic between DHV/CSI (#24896)

When we implemented CSI, the types of the fields for access mode and attachment
mode on volume requests were defined with a prefix "CSI". This gets confusing
now that we have dynamic host volumes using the same fields. Fortunately the
original was a typedef on string, and the Go API in the `api` package just uses
strings directly, so we can change the name of the type without breaking
backwards compatibility for the msgpack wire format.

Update the names to `VolumeAccessMode` and `VolumeAttachmentMode`. Keep the CSI
and DHV specific value constant names for these fields (they aren't currently
1:1), so that we can easily differentiate in a given bit of code which values
are valid.

Ref: https://github.com/hashicorp/nomad/pull/24881#discussion_r1920702890

command/agent/job_endpoint.go

@@ -1336,8 +1336,8 @@ func ApiTgToStructsTG(job *structs.Job, taskGroup *api.TaskGroup, tg *structs.Ta
 				ReadOnly:       v.ReadOnly,
 				Sticky:         v.Sticky,
 				Source:         v.Source,
-				AttachmentMode: structs.CSIVolumeAttachmentMode(v.AttachmentMode),
-				AccessMode:     structs.CSIVolumeAccessMode(v.AccessMode),
+				AttachmentMode: structs.VolumeAttachmentMode(v.AttachmentMode),
+				AccessMode:     structs.VolumeAccessMode(v.AccessMode),
 				PerAlloc:       v.PerAlloc,
 			}