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": "63088aea-420c-4c4a-8cf0-8c34476255aa",
    "type": "incident_sub_statuses",
    "attributes": {
      "sub_status_id": "3d4a5ff3-33d8-4d05-b30b-0579db661121",
      "incident_id": "a1741727-564b-4734-8b7f-a6f9bfc09d8b",
      "assigned_at": "2025-11-18T15:39:16.000-08:00",
      "assigned_by_user_id": null
    },
    "relationships": {
      "sub_status": {
        "data": {
          "id": "3d4a5ff3-33d8-4d05-b30b-0579db661121",
          "type": "sub_statuses"
        }
      },
      "assigned_by_user": {
        "data": null
      }
    }
  }
}