creates an override shift

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

{
  "data": {
    "id": "51c9eee5-e5f7-4950-8023-c027600a156d",
    "type": "shifts",
    "attributes": {
      "schedule_id": "8f8b4a3e-f2c4-4a28-9420-8ca945735d2a",
      "rotation_id": null,
      "user_id": 444,
      "starts_at": "2025-12-18T10:39:52.000-08:00",
      "ends_at": "2025-12-19T08:39:52.000-08:00",
      "is_override": true
    },
    "relationships": {
      "shift_override": {
        "data": {
          "id": "05597329-6455-4290-982d-40568ff53299",
          "type": "shift_overrides"
        }
      },
      "user": {
        "data": {
          "id": "444",
          "type": "users"
        }
      },
      "assignee": {
        "data": {
          "id": "444",
          "type": "users"
        }
      }
    }
  }
}