Add method to get the callbacks from a characteristic.

This is useul to be able to unit/integration test the behavior of a
characteristic.

src/NimBLECharacteristic.h

@@ -65,6 +65,8 @@ public:
     std::string       toString();
 
     void              setCallbacks(NimBLECharacteristicCallbacks* pCallbacks);
+    NimBLECharacteristicCallbacks*
+                      getCallbacks();
 
     void              indicate();
     void              notify(bool is_notification = true);