Add support for escape hatch listeners to Ingress Gateways

Ingress gateways are unable to utilize any of the existing listener escape hatch overrides.

At the moment, [their upstream config is generated](https://github.com/hashicorp/consul/blob/master/agent/proxycfg/state.go#L1478) from the `GatewayService` type which comes from the ingress gateway config entry. One potential solution is to accept a listener override in the [ingress listener definition](https://www.consul.io/docs/agent/config-entries/ingress-gateway#listeners).

A caveat with using that escape hatch is that we need to document that users need to ensure the [route_config_name is set to the listener port](https://github.com/hashicorp/consul/blob/master/agent/xds/testdata/listeners/ingress-http-multiple-services.envoy-1-12-x.golden#L29), since this is the key we use for RDS routes.

For additional context see this discuss thread: https://discuss.hashicorp.com/t/using-escape-hatch-overrides-with-ingress-gateway/10834