The List Volumes API was originally written for CSI but assumed we'd have future
volume types, dispatched on a query parameter. Dynamic host volumes uses this,
but the resulting code has host volumes concerns comingled in the CSI volumes
endpoint. Refactor this so that we have a top-level `GET /v1/volumes` route that's
shared between CSI and DHV, and have it dispatch to the appropriate handler in
the type-specific endpoints.
Ref: https://github.com/hashicorp/nomad/pull/24479
