Single Cluster Regional LB (gke-l7-rilb) https with Private Service Connect example

[ap0phi5]

In the same form as #59,
could you possibly add an example of how to connect a Service Attachment to GKE Gateway API Controller as a PSC producer?

With the L4 nginx Ingress Controller, this was previously done with a ServiceAttachment resource:

apiVersion: networking.gke.io/v1beta1
kind: ServiceAttachment
metadata:
 name: emoji-sa
 namespace: default
spec:
 connectionPreference: ACCEPT_AUTOMATIC
 natSubnets:
 - gke-nat-subnet
 proxyProtocol: false
 resourceRef:
   kind: Service
   name: gke-l4-psc

This is for a Private GKE Cluster in a service project with access to a Shared VPC.