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": "89cb3e58-abbc-49c2-8cdb-baa2783b7857",
    "type": "incident_sub_statuses",
    "attributes": {
      "sub_status_id": "68476688-d4f2-4bc8-a4fa-0140760129a7",
      "incident_id": "c516d8ab-c4b5-4576-b325-be12d7d3613f",
      "assigned_at": "2025-12-01T14:19:30.000-08:00",
      "assigned_by_user_id": null
    },
    "relationships": {
      "sub_status": {
        "data": {
          "id": "68476688-d4f2-4bc8-a4fa-0140760129a7",
          "type": "sub_statuses"
        }
      },
      "assigned_by_user": {
        "data": null
      }
    }
  }
}