add return eventSubId (#428)

Author: ywy2090

sdk-service/src/main/java/org/fisco/bcos/sdk/eventsub/EventSubscribe.java

@@ -55,7 +55,7 @@ static EventSubscribe build(Client client) throws JniException {
      * @param callback the EventCallback instance
      * @return registerId of event
      */
-    void subscribeEvent(EventSubParams params, EventSubCallback callback);
+    String subscribeEvent(EventSubParams params, EventSubCallback callback);
 
     /**
      * Unsubscribe events

sdk-service/src/main/java/org/fisco/bcos/sdk/eventsub/EventSubscribeImp.java

@@ -76,24 +76,24 @@ public void setConfigOption(ConfigOption configOption) {
     }
 
     @Override
-    public void subscribeEvent(EventSubParams params, EventSubCallback callback) {
+    public String subscribeEvent(EventSubParams params, EventSubCallback callback) {
 
         if (!params.checkParams()) {
             callback.onReceiveLog("", EventSubStatus.INVALID_PARAMS.getStatus(), null);
-            return;
+            return "";
         }
 
         String strParams = null;
         try {
             strParams = objectMapper.writeValueAsString(params);
         } catch (JsonProcessingException e) {
             logger.error("e: ", e);
-            return;
+            return "";
         }
 
         logger.info("EventSub subscribeEvent, params: {}", params);
 
-        eventSubscribe.subscribeEvent(
+        return eventSubscribe.subscribeEvent(
                 groupId,
                 strParams,
                 new EventSubscribeCallback() {