Support multiple VirtualService in Istio canary strategy

[shakti-das]

Summary

The current Istio canary strategy allows specifying only a single VirtualService. This is not sufficient for advanced setups where the routes are divided into multiple VirtualService resources - like north-south route VS, east-west route VS, and delegate VS.

Use Cases

In Salesforce, we use multiple VirtualService resources linked to a single service. Each service port also gets its own specialized delegate VirtualService. The blue/green route for our use-case would actually stay in the multiple delegates. When the Service undergoes a blue/green rollout, we want to update all the linked VirutalService resources.

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[jessesuen]

Duplicate of #1100