New contribution: ahtxx device driver library

Greetings community!

I would like to contribute a Mongoose OS adaptation for a library: AHTXX device driver library

Overview: Adoption created based on the source code https://github.com/enjoyneering/AHTxx. Full documentation provided in readme file. The library successfully tested on ESP8266.

Link: https://github.com/grand87/AHTxx

Any feedback and suggestions are welcomed!