Enable RDC link Status feature

1.add link status APIs
   2.Add link status example for link status API usage

server/include/rdc/rdc_api_service.h

@@ -152,11 +152,14 @@ class RdcAPIServiceImpl final : public ::rdc::RdcAPI::Service {
   ::grpc::Status UnRegisterPolicy(::grpc::ServerContext* context,
                                   const ::rdc::UnRegisterPolicyRequest* request,
                                   ::rdc::UnRegisterPolicyResponse* reply) override;
-                                  
+
   ::grpc::Status GetTopology(::grpc::ServerContext* context,
                              const ::rdc::GetTopologyRequest* request,
                              ::rdc::GetTopologyResponse* reply) override;
 
+  ::grpc::Status GetLinkStatus(::grpc::ServerContext* context, const ::rdc::Empty* request,
+                               ::rdc::GetLinkStatusResponse* reply) override;
+
   ::grpc::Status SetHealth(::grpc::ServerContext* context, const ::rdc::SetHealthRequest* request,
                            ::rdc::SetHealthResponse* reply) override;