bugfix - Added Missing Parameter for Subscribe Request in Cpp Client

Expected Behavior: Corelink cpp client sends a request of format {'function': 'subscribe', 'receiverID': _, 'streamIDs': [], 'token': _}

Actual Behavior:

• The function parameter is missing Corelink cpp client sends a request of format {'receiverID': _, 'streamIDs': [], 'token': _}

Fix

• Use json.append to add the function param value.