Add /healthcheck endpoints to facilitate deploying to k8s

[jeffgran-dox]

Adds /healthcheck endpoints to the app and webhook servers so that k8s deployment can check when the node is up.

[mattmoor]

Given that this is a GRPC service, I think I'd rather see us register the healthpb service and use a grpc probe, which thanksfully k8s now has.

I think it basically entails importing:

	"google.golang.org/grpc/health"
	healthpb "google.golang.org/grpc/health/grpc_health_v1"

and then doing something like this to register the service:

	// Register health check service
	healthServer := health.NewServer()
	healthpb.RegisterHealthServer(d.Server, healthServer)