Add kustomization.yaml to istio-ingressgateway sample resources

[mark-j-chilvers]

I'm working on updating our Edge to Mesh guide along with @theemadnes . We would like to refer to this repo as a sensible starting point for ASM Ingress Gateway deployment manifests and avoid duplication within the guide. We would then make necessary patches via Kustomize. In order to accomplish this it would be great if a kustomization.yaml file could be added listing the resources in the istio-ingressgateway folder.

[zerobfd]

I would think that https://github.com/GoogleCloudPlatform/anthos-service-mesh-samples would be a better place, is there a reason why this one makes more sense?