Get usage alerts for a subscription

curl --request GET \
  --url {schemes}://api.upsun.com/alerts/subscriptions/{subscriptionId}/usage \
  --header 'Authorization: Bearer <token>'

{
  "available": [
    {
      "id": "<string>",
      "active": true,
      "alerts_sent": 123,
      "last_alert_at": "2023-11-07T05:31:56Z",
      "updated_at": "2023-11-07T05:31:56Z",
      "config": {}
    }
  ],
  "current": [
    {
      "id": "<string>",
      "active": true,
      "alerts_sent": 123,
      "last_alert_at": "2023-11-07T05:31:56Z",
      "updated_at": "2023-11-07T05:31:56Z",
      "config": {}
    }
  ]
}