Implement http readiness for app

Many k8s apps - like Istio testapp - use http readiness check. Cloudrun readiness is determined by app listening on the port. Migrating to cloudrun would require code changes to avoid 503 on startup.

With the mesh, we have the code to do http readiness checks ( we do it for VMs ) - just need to configure it and call it.

This is required at least for passing the e2e tests without flakes.