Support host header rewrite for the Kuma builtin gateway

For external services which are themselves a load balancer (for example an K8s Ingress Controller) that can route requests using the host header, the customer needs host headers to be swapped with the hostname of the upstream host chosen by the cluster manager, i.e. by setting the {{auto_host_rewrite}} on the route (on the MeshGatewayRoute resource)

Without this we can only route directly to services but not via many common middle boxes.