Userspace Proxy: allow check for endpoints on svc

This commit adds a method to the `LoadBalancer` interface in the
userspace proxy which allows consumers of the `LoadBalancer` to check if
it thinks a given service has endpoints available.

pkg/proxy/userspace/loadbalancer.go

@@ -31,4 +31,5 @@ type LoadBalancer interface {
 	NewService(service proxy.ServicePortName, sessionAffinityType api.ServiceAffinity, stickyMaxAgeMinutes int) error
 	DeleteService(service proxy.ServicePortName)
 	CleanupStaleStickySessions(service proxy.ServicePortName)
+	ServiceHasEndpoints(service proxy.ServicePortName) bool
 }