Support proxy canary in Envoy gateway

*Description*:
It would beneficial for users to be able to rollout proxy config change to canary service first prior to wide fleet rollout. EG could be managing multiple envoyproxy services (stable and canary) without resource duplications (users dont need to explicitly configure & duplicate resources for canary gateway class, gateway, envoyproxy config etc). Another idea is to have both canary EG (control plane) service talking to canary envoyproxies and stable EG+envoyproxies and deployment/rollout that progressively deploys to canary first and then to stable.

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