Design generic TrafficRouting mechanism to meet Istio, Alb or custom traffic regulation

[zmberg]

What would you like to be added:

Rollout Support Traffic Graying for Istio Envoy, as follows:

apiVersion: rollouts.kruise.io/v1alpha1
kind: Rollout
metadata:
  name: rollouts-demo
spec:
  strategy:
    canary:
      trafficRoutings: 
      - service: echoserver  # required
         ## istio, nginx, alb
         type: istio
         CustomTraffic:
             apiVersion: xxxx
             kind: istio
             name: xxxx

[vjoke]

When will this feature be ready for traffic routing of Istio?