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.