Traffic Mirroring (Shadowing)

**Tell us about your request**
This feature request is for implementing traffic mirroring (also referred to as shadowing). Traffic mirroring allows one service to send the same traffic to more than one upstream service while still only using a single upstream service for the authoritative response. Other services which are receiving mirrored traffic can be tested for bugs and performance regressions prior to serving real traffic and becoming the authoritative upstream.

**Which integration(s) is this request for?**
All

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
When working with microservices, developers and infrastructure engineers often need to test their new versions against real traffic before shifting all live traffic over to the new version. This increases confidence in code changes and allows teams to find bugs during periods of change.

**Are you currently working around this issue?**
App Mesh does not currently supporting traffic mirroring, so teams may work around the issue by replaying old traffic patterns from previous logs collected.

**Additional context**
Envoy Proxy supports [traffic mirroring](https://www.envoyproxy.io/docs/envoy/latest/api-v2/api/v2/route/route.proto#envoy-api-msg-route-routeaction-requestmirrorpolicy) on routes.