CSI: display size limits on plugins via GetCapacity RPC

Some CSI plugins support the Controller [`GetCapacity`](https://github.com/container-storage-interface/spec/blob/master/spec.md#getcapacity) RPC, which sets a maximum and minimum size for volumes. The plugin already validates these when we create a volume (#10165), but it would be nice to give the user a chance to set the right values before they try to create a volume. We can call `GetCapacity` as part of the `nomad plugin status :plugin_id` command to display these values (maybe only if the `-verbose` flag is set for performance).