Prepare handing over of sync callback via update function to be phased out

Currently the callback for a 'onSync' event is passed via update function. Since the signature of this sync function 'void(*fn)(void)' is not compatible with the cloud event sync function signature 'void(*fn)(void *)' it can not be registered via 'addCallback' as a normal cloud event sync function. Since the future way of adding callbacks is done via the 'addCallback' function we prepare the update functions in the wway shown in this commit for removing the capability of adding a sync callback function in the near future.

Author: aentinger

Diff for: src/ArduinoIoTCloud.h

@@ -83,10 +83,16 @@ class ArduinoIoTCloudClass {
     int  connect();
     bool disconnect();
 
-    void update(CallbackFunc onSyncCompleteCallback = NULL);
+    inline void update() {
+      update(NULL);
+    }
+    void update(CallbackFunc onSyncCompleteCallback) __attribute__((deprecated)); /* Attention: Function is deprecated - use 'addCallback(ArduinoIoTCloudConnectionEvent::SYNC, &onSync)' for adding a onSyncCallback instead */
 
     // defined for users who want to specify max reconnections reties and timeout between them
-    void update(int const reconnectionMaxRetries, int const reconnectionTimeoutMs, CallbackFunc onSyncCompleteCallback = NULL);
+    inline void update(int const reconnectionMaxRetries, int const reconnectionTimeoutMs) {
+      update(reconnectionMaxRetries, reconnectionTimeoutMs, NULL);
+    }
+    void update(int const reconnectionMaxRetries, int const reconnectionTimeoutMs, CallbackFunc onSyncCompleteCallback) __attribute__((deprecated)); /* Attention: Function is deprecated - use 'addCallback(ArduinoIoTCloudConnectionEvent::SYNC, &onSync)' for adding a onSyncCallback instead */
 
     int connected();
     // Clean up existing Mqtt connection, create a new one and initialize it