gRPC Mirroring

### Is your feature request related to a problem?

Envoy supports request mirroring, which sends the same event to a primary Envoy cluster and also forwards it to another secondary cluster, configured either statically or dynamically.

This feature would be handy for testing and validation, such as migrating from one service to another or checking for regressions between two versions.

### Describe the solution you'd like

The solution we are looking for is the ability to mirror the same requests sent to two different gRPC services based on a static definition, like in here: https://www.envoyproxy.io/docs/envoy/latest/start/sandboxes/route-mirror.html

### Describe alternatives you've considered

One alternative would be to proxy our traffic through an Envoy deployment, but this would mean an extra hop in our system, as we are doing client-side load balancing right now.

Beginne mit der verknüpften Envoy route-mirror-Dokumentation und überprüfe das clientseitige Load-Balancing-Verhalten des Repositorys, das im Issue als aktueller Traffic-Pfad bezeichnet wird. Die Aufgabe ist abgeschlossen, wenn das statische Mirroring von Anfragen an zwei gRPC-Dienste definiert und implementiert ist und während des Designs Tests und Konfigurationspunkte festgelegt wurden.

Vom Indexierungsmodell aus dem Issue-Text verfasst.