Support Envoy's Slow Start Mode via Service Resolver

Envoy natively supports Slow Start mode https://www.envoyproxy.io/docs/envoy/latest/intro/arch_overview/upstream/load_balancing/slow_start but this option is not available in Consul Service Mesh. Service Resolver is likely a good place for this configuration as it already supports other envoy cluster configuration like lb_policy or least_request_lb_config.

I tried to enable this using EnvoyExtension via ProxyDefaults but it does not apply for API Gateway.