Creates a sub-status assignment

curl --request POST \
  --url https://api.rootly.com/v1/incidents/{incident_id}/sub_statuses \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/vnd.api+json' \
  --data '{
  "data": {
    "type": "incident_sub_statuses",
    "attributes": {
      "sub_status_id": "<string>",
      "assigned_at": "<string>",
      "assigned_by_user_id": 123
    }
  }
}'

{
  "data": {
    "id": "9714d032-3a7b-4a80-a961-887da51a7d63",
    "type": "incident_sub_statuses",
    "attributes": {
      "sub_status_id": "73bc1aa0-d5d6-4e9a-8f22-99830b7bd2ef",
      "incident_id": "f31f6d93-c917-4119-9bbd-654ec4c1842e",
      "assigned_at": "2025-08-02T12:38:53.000-07:00",
      "assigned_by_user_id": null
    },
    "relationships": {
      "sub_status": {
        "data": {
          "id": "73bc1aa0-d5d6-4e9a-8f22-99830b7bd2ef",
          "type": "sub_statuses"
        }
      },
      "assigned_by_user": {
        "data": null
      }
    }
  }
}