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": "4b3dafc2-7236-4e10-89c9-49818e161241",
    "type": "incident_sub_statuses",
    "attributes": {
      "sub_status_id": "ed9f3773-b4d8-4c21-a1b6-f624c1399c78",
      "incident_id": "0e0b0e14-cc3e-4575-b231-b8a4e5f94d2d",
      "assigned_at": "2025-09-15T22:55:05.000-07:00",
      "assigned_by_user_id": null
    },
    "relationships": {
      "sub_status": {
        "data": {
          "id": "ed9f3773-b4d8-4c21-a1b6-f624c1399c78",
          "type": "sub_statuses"
        }
      },
      "assigned_by_user": {
        "data": null
      }
    }
  }
}