Creates a schedule rotation user

curl --request POST \
  --url https://api.rootly.com/v1/schedule_rotations/{schedule_rotation_id}/schedule_rotation_users \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/vnd.api+json' \
  --data '{
  "data": {
    "type": "schedule_rotation_users",
    "attributes": {
      "user_id": 123,
      "position": 123
    }
  }
}'

{
  "data": {
    "id": "219ea627-6057-4caa-bd8d-69df5ae5124f",
    "type": "schedule_rotation_users",
    "attributes": {
      "schedule_rotation_id": "45bc1ad8-ff7a-4042-b2cb-cde5cdfc0fff",
      "user_id": 451,
      "position": 2,
      "created_at": "2025-10-08T19:36:45.943-07:00",
      "updated_at": "2025-10-08T19:36:45.943-07:00"
    }
  }
}