The API is missing values for `ReadAllocs` and `WriteAllocs` fields, resulting
in allocation claims not being populated in the web UI. These fields mirror
the fields in `nomad/structs.CSIVolume`. Returning a separate list of stubs
for read and write would be ideal, but this can't be done without either
bloating the API response with repeated full `Allocation` data, or causing a
panic in previous versions of the CLI.
The `nomad/structs` fields are persisted with nil values and are populated
during RPC, so we'll do the same in the HTTP API and populate the `ReadAllocs`
and `WriteAllocs` fields with a map of allocation IDs, but with null
values. The web UI will then create its `ReadAllocations` and
`WriteAllocations` fields by mapping from those IDs to the values in
`Allocations`, instead of flattening the map into a list.
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