https://firebase.google.com/docs/cloud-messaging/ios/live-activity

With this field and this configuration I am able to send LiveActivity start messages:

```
messaging.Message(
        apns=messaging.APNSConfig(
            live_activity_token=live_activity_token,
            payload=messaging.APNSPayload(
                aps=messaging.Aps(
                    alert=messaging.ApsAlert(
                        title="Live Activity Started",
                        body="Your live activity has been started",
                    ),
                    custom_data = {
                        "input-push-token": 1,
                        "event": "start",
                        "timestamp": int(time.time()),
                        "content-state": {
                            "myAttribute": False
                        },
                        "attributes-type": "LiveActivityAttributes",
                        "attributes": {
                            "myAttribute": False
                        },
                    }
                ),
                headers={
                    "apns-priority": "5",
                },
            ),
        ),
        token=cloud_messaging_token,
    )
```