[GCP] Allow Ingress ILB to define a specific subnet

1. Proposed title of this feature request
[GCP] Allow Ingress ILB to define a specific subnet

2. What is the nature and description of the request?
When creating an OpenShift cluster in Google Cloud, the cluster-ingress-operator creates the service "router-default" that is used to create an ILB in Google cloud for the router.
The service is managed by the IngressController, and the object do not support any parameter to define the a specific subnet to which the ILB should be connected

GCE supports the definition of the custom subnet via the annotation `networking.gke.io/internal-load-balancer-subnet` https://cloud.google.com/kubernetes-engine/docs/how-to/internal-load-balancing#lb_subnet

https://github.com/kubernetes/ingress-gce/blob/c4fd082b87afdc5fa294920477eca368db269d87/vendor/k8s.io/legacy-cloud-providers/gce/gce_annotations.go#L61-L64

3. Why does the customer need this? (List the business requirements here)

• Customer needs to place one or more ingress router into dedidated subnets for security reasons and requirements
• Right now as a workaround is possible to annotate the Ingress service but this will complicate the management and can lead to possible issues

4. List any affected packages or components.

• Ingress Controller
• cluster-ingress-operator