creates an shadow configuration

curl --request POST \
  --url https://api.rootly.com/v1/schedules/{schedule_id}/on_call_shadows \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/vnd.api+json' \
  --data '{
  "data": {
    "type": "on_call_shadows",
    "attributes": {
      "shadowable_type": "User",
      "shadowable_id": "<string>",
      "shadow_user_id": 123,
      "starts_at": "2023-11-07T05:31:56Z",
      "ends_at": "2023-11-07T05:31:56Z"
    }
  }
}'

{
  "data": {
    "id": "9dd6b768-0de1-49c4-afc5-5c9e0e068163",
    "type": "on_call_shadows",
    "attributes": {
      "schedule_id": "6bc5a082-71ab-4798-b0b4-5bd6ce162c03",
      "shadowable_type": "User",
      "shadowable_id": "359",
      "shadow_user_id": 358,
      "starts_at": "2025-09-16T17:32:51.107-07:00",
      "ends_at": "2025-09-17T17:32:51.107-07:00",
      "created_at": "2025-09-14T17:32:51.176-07:00",
      "updated_at": "2025-09-14T17:32:51.176-07:00"
    }
  }
}