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_id": "<string>",
      "shadow_user_id": 123,
      "starts_at": "2023-11-07T05:31:56Z",
      "ends_at": "2023-11-07T05:31:56Z"
    }
  }
}
'

{
  "data": {
    "id": "<string>",
    "type": "on_call_shadows",
    "attributes": {
      "schedule_id": "<string>",
      "shadowable_id": "<string>",
      "shadow_user_id": 123,
      "starts_at": "2023-11-07T05:31:56Z",
      "ends_at": "2023-11-07T05:31:56Z",
      "created_at": "<string>",
      "updated_at": "<string>"
    }
  },
  "included": [
    {
      "id": "<string>",
      "type": "<string>",
      "attributes": {},
      "relationships": {}
    }
  ]
}