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
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