Adding length limit to cloud value in CloudWrapperString.h

**Issue:** If a user sends a too-long string in the Arduino IoT Cloud textbox area, they freeze their Arduino device.

**Suggested mitigation:** add a string length limit to cloud value in [CloudWrapperString.h](https://github.com/arduino-libraries/ArduinoCloudThing/blob/master/src/types/CloudWrapperString.h)