Modify LoRaChat to use it as a library

[NilLlisterri]

I've made a few changes to transform the application into a library:

• Moved the Wi-Fi credentials and MQTT connection details out of config.h so plaform.io does not override the user defined credentials automatically importing the file.
• Created the LC (LoRaChat) class to make the library user-friendly.
• Moved the demo code to the examples' folder. This way, platform.io won't automatically include the files.
• Created the library.json file to list the library dependencies and have them installed automatically when using the library.

TODO: Find an alternative to declare the application ports when registering the application.

[NilLlisterri]

Hi! I've added a new commit, whose main contribution is to add a virtual destructor to the DataMessageGeneric class. This was needed to solve a memory leak caused by the deletion of an instance of a class that inherited from DataMessageGeneric.

E.g. when MqttService::processReceivedMessageFromMQTT deletes the message at the end.