Add AHT21 Humidity & Temperature sensor

[afloarea]

Add AHT21 Humidity & Temperature sensor

You may have this in the works already but I got an AHT21 sensor which I found was not already in the list of supported devices so I thought I'd give this a shot and from what I can tell, it seems to be working fine.

Some additional notes:

• In my case, the sensor always returned ok status 0x18 and the initRegistries call was never needed.
• The datasheet points to an "official website implementation" for the registry initialization logic, so I just adapted what was there

Thank you for maintaining this awesome project 🙏.

[afloarea]

Hello, I have updated the PR according to your suggestions and I await for your feedback. Thanks