Derive Gateway addresses from Service status instead of hardcoded 127.0.0.1

Replace the hardcoded 127.0.0.1 Gateway address with dynamic address
derivation from cluster state. The address resolution follows this
priority order:

1. Static addresses from gateway.spec.addresses (user-specified)
2. LoadBalancer ingress IPs/hostnames from the data plane Service
3. Node internal IP (for NodePort/hostPort, e.g. Kind clusters)
4. Service ClusterIP

When no address is available (e.g. first reconciliation before the
Service exists), the Gateway is set to Accepted=True but
Programmed=False with reason AddressNotAssigned.

This is required for the GatewayWithAttachedRoutes conformance test,
which expects real addresses from the Service status rather than
localhost.

Author: RobbieMcKinstry