Envoy factory registry

Envoy is an incredibly flexible proxy server. Almost every aspect of the processing chain from the listeners all the way to the management of the upstream connections is configurable. This is even more impressive given that the configuration can be changed on the fly (by reloading a file or via the xDS API) without restarting the server. Moreover, we are not restricted to Envoy's built-in features but can add our own components with their custom configuration. How does Envoy pull this off given that C++ is