Publish refactoring (#298)

* factoring of the publishing function
* integration of json publishing to MQTT
* receiving and transcode json messages from mqtt
* replace the TRACE runtime condition by a buildtime one so as to decrease program size
* remove retainFlag parameter not necessary for the moment
* Increase the scope of tests for arduino mega and esp32
* Update arduinojson library to 5.13.3
* add macros to choose between simple or json publishing or both
* differentiate build test for ATmega2560
* correct wrong sprintf definition
* change IDE version for continuous integration
* let default warning level in arduino ide
* correct pins for ESP32
* reorganize user_config

lib/ArduinoJson/examples/StringExample/StringExample.ino

@@ -64,11 +64,11 @@ void loop() {
 // See also
 // --------
 //
-// The website arduinojson.org contains the documentation for all the functions
+// https://arduinojson.org/ contains the documentation for all the functions
 // used above. It also includes an FAQ that will help you solve any problem.
-// Please check it out at: https://arduinojson.org/
 //
 // The book "Mastering ArduinoJson" contains a quick C++ course that explains
 // how your microcontroller stores strings in memory. On several occasions, it
 // shows how you can avoid String in your program.
-// Please check it out at: https://arduinojson.org/book/
+// Learn more at https://arduinojson.org/book/
+// Use the coupon code TWENTY for a 20% discount ❤❤❤❤❤