Reimplement support for Security Command Center

The existing [Google Cloud Security Command Center integration](https://console.cloud.google.com/marketplace/details/cloudflare/cloudflare-security-events) is deprecated (https://github.com/cloudflare/cloudflare-gcp/tree/master/security-events). Now that Firewall Events can be delivered as an independent logstream, previous CPU and memory constraints imposed by Cloud Functions are no longer an issue.