Add method to set Client*

Author: pennam

Diff for: src/MqttClient.h

@@ -50,6 +50,7 @@ class MqttClient : public Client {
   void onMessage(MessageCallback callback);
 #else
   inline void setClient(Client& client) { _client = &client; }
+  inline void setClient(Client* client) { _client = client; }
   void onMessage(void(*)(int));
 #endif