healthcheck: add support at the load-balancers of services level

Co-authored-by: Dmitry Sharshakov <d3dx12.xx@gmail.com>
Co-authored-by: Julien Salleyron <julien.salleyron@gmail.com>
Co-authored-by: Jean-Baptiste Doumenjou <925513+jbdoumenjou@users.noreply.github.com>
Co-authored-by: Romain <rtribotte@users.noreply.github.com>
Co-authored-by: Tom Moulard <tom.moulard@traefik.io>

pkg/middlewares/emptybackendhandler/empty_backend_handler_test.go

@@ -48,6 +48,10 @@ type healthCheckLoadBalancer struct {
 	amountServer int
 }
 
+func (lb *healthCheckLoadBalancer) RegisterStatusUpdater(fn func(up bool)) error {
+	return nil
+}
+
 func (lb *healthCheckLoadBalancer) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	w.WriteHeader(http.StatusOK)
 }