Support Custom Envoy configuration on Consul API Gateway

[shashank201192]

We are looking for a way to customise upstream cluster configuration(enable slow start mode) on Consul API Gateway which is not natively supported by Consul already. Proxy default based EnvoyExtension configurations are not applied Consul API Gateway.
Similar discussion started at https://discuss.hashicorp.com/t/configuring-low-level-envoy-proxy-settings-in-the-consul-api-gateway/42356 but only ticket related to Ingress-Gateway is created. Hence creating this ticket to track this request for Consul API Gateway.

While adding support for slow start mode on HTTP route config entry would work for us but this request is more generic to customise Envoy config in general on API Gateway.