Register a callback function with ArduinoCloudThing (#93)

This allows class ArduinoCloudThing to obtain a 'epoch' timestamp which is used during synchronisation and which has been implemented so far via an 'extern' dependency to RTCZero. Since this kind of hidden dependency (basically a global object across 2 separate Arduino libraries) is very brittle the depdency is made clear by explicitly registering a function to provide the required timestamp.

Author: aentinger

src/ArduinoIoTCloudTCP.cpp

@@ -140,6 +140,7 @@ int ArduinoIoTCloudTCP::begin(String brokerAddress, uint16_t brokerPort) {
   mqttClientBegin();
 
   Thing.begin();
+  Thing.registerGetTimeCallbackFunc(getTime);
   return 1;
 }