Support deployment of proxy without listeners

*Description*:
When using merge-gateways, the proxy can be eagerly deployed based on the gatewayclass, before any gateway is available. This is useful for users that want to ensure that the proxy is successfully deployed and discover LB addresses prior to configuring the proxy with resources like `Gateway`, `xRoute`, etc.

However, since a service without ports cannot be created, this functionality is currently disabled: https://github.com/envoyproxy/gateway/issues/3044.

Some options to consider:
- Render the service conditionally, only if listeners (ports) exist. Other resources (deployment, etc.) can still be provisioned.
- Allow EnvoyProxy to explicitly define "default" service ports (e.g. 80, 443). The default target will be overriden once a listener exists. This can either be a proper API (preferred) or work through a patch mechanism.