Add dashboard information in `scw info`

[moul]

$ curl -X GET -H "X-Auth-Token: 00000000-0000-1000-0000-000000000000" -H "Content-Type: application/json" https://api.scaleway.com/dashboard

{
  "dashboard": {
    "volumes_count": 201,
    "running_servers_count": 103,
    "images_count": 13,
    "snapshots_count": 111,
    "servers_count": 147,
    "ips_count": 12
  }
}