Support per-Gateway LoadBalancer annotations

[rg0now]

Currently the recommended way to customize the LoadBalancers created by STUNner is through adding annotations to the GatewayConfig spec.loadBalancerServiceAnnotations field. This, however, is global, in that it affects all Gateways, it does not make it possible to stop STUNner from exposing a Gateway, and it does not make it possible to add labels, only annotations.

This issue is a placeholder to add another way for controlling LoadBalancers and to discuss and implement this behavior. In particular, Istio implements a policy where annotations/labels can be added to each Gateway and these will be automatically copied into the Kubernetes services created for that Gateway.

The particular behavior (converted to STUNner) would be as follows:

• Annotations and labels on the Gateway will be copied into the Service. This allows configuring things such as Internal load balancers that read from these fields.
• STUNner offers an additional annotation to configure the generated resources: adding a special annotation stunner.l7mp.io/service-type controls the Service.spec.type field of the Service. For example, set to ClusterIP to not expose the service externally. The default is LoadBalancer.