csi/api: populate ReadAllocs/WriteAllocs fields (#9377)

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.

website/pages/api-docs/volumes.mdx

@@ -230,6 +230,10 @@ $ curl \
       "ModifyTime": 1495747371794276400
     }
   ],
+  "ReadAllocs": {
+    "a8198d79-cfdb-6593-a999-1e9adabcba2e": null
+  },
+  "WriteAllocs": {},
   "Schedulable": true,
   "PluginID": "plugin-id1",
   "Provider": "ebs",