CSI: report specific capabilities w/ plugin status output

[tgross]

Right now the only good way to get the specific capabilities that a CSI plugin supports is via the Container Storage Client (csi):

$ sudo csc --endpoint /var/nomad/client/csi/controller/cephrbd/csi.sock controller get-capabilities
&{type:CREATE_DELETE_VOLUME }
&{type:CREATE_DELETE_SNAPSHOT }
&{type:CLONE_VOLUME }
&{type:EXPAND_VOLUME }

The nomad plugin status -verbose flag just expands the UUIDs:

$ nomad plugin status -verbose cephrbd
ID                   = cephrbd
Provider             = rbd.csi.ceph.com
Version              = canary
Controllers Healthy  = 1
Controllers Expected = 1
Nodes Healthy        = 1
Nodes Expected       = 1

Allocations
ID                                    Eval ID                               Node ID                               Node Name   Task Group  Version  Desired  Status   Created                    Modified
8b2eb95a-32e2-c7d4-f939-b968137e6c81  1fc9d37b-ed78-9093-5ef7-2836e3e39051  23542ed8-4b49-35a5-f74d-892d980cecf0  standalone  cephrbd     0        run      running  2021-03-24T10:13:54-04:00  2021-03-24T10:14:05-04:00
2ad097b8-9ff0-ef9b-f42a-f6173e89f8cd  67331428-fcf5-0912-fdc1-7904608a7403  23542ed8-4b49-35a5-f74d-892d980cecf0  standalone  cephrbd     0        run      running  2021-03-24T10:13:55-04:00  2021-03-24T10:14:01-04:00

We could add a list of supported capabilities to the API and report them here.

[tgross]

Copied context from #11859:

While working on debugging #10927 with some new-to-me CSI plugins, I found that being able to query the plugin capabilities as part of the output of nomad plugin status :plugin_id would be useful (perhaps only with the -verbose flag). The information is available in the state store, so it's just a matter of plumbing through all the RPC response, the HTTP API, and then the CLI.

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