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": "a380e3dd-644e-4709-a6dd-3693027a33d7",
    "type": "schedule_rotation_users",
    "attributes": {
      "schedule_rotation_id": "239f50d9-2802-49c7-bd49-9d481ffbb4e4",
      "user_id": 518,
      "position": 2,
      "created_at": "2025-12-03T15:48:21.786-08:00",
      "updated_at": "2025-12-03T15:48:21.786-08:00"
    }
  }
}