optimized returned reply in subscribe method

Author: kyooosukedn

Diff for: eventmesh-sdks/eventmesh-sdk-rust/src/grpc/grpc_consumer.rs

@@ -161,12 +161,11 @@ impl EventMeshGrpcConsumer {
                             ProtocolKey::SUB_MESSAGE_TYPE.to_string(),
                             PbCloudEventAttributeValue {
                                 attr: Some(PbAttr::CeString(
-                                    SubscriptionReply::SUB_TYPE.to_string(),
+                                    ProtocolKey::SUB_REPLY_MESSAGE.to_string(),
                                 )),
                             },
                         );
-                        received.data =
-                            Some(PbData::TextData(serde_json::to_string(&reply).unwrap()));
+                        received.data = None;
                         let _ = keeper.sender.send(received).await;
                     }
                 } else {