feat: New Airthings input plugin

### Use Case

Airthings (https://www.airthings.com/) is supplying an API that allows you to access the data collected from sensors that indoor air quality and mainly the gas radon.
We would like to have a Telegraf input plugin that exposes Airthings metrics
Expose metrics for Airthings in Telegraf collector to enable processing and aggregation of Airthings sensor data.

### Expected behavior

In the near future, I will propose PR which adds a new plugin I have created. The plugin will be able to fetch Airthings metrics from the Airthings API.

### Actual behavior

Currently, it is not possible to gather and expose Airthings metrics.

### Additional info

Airthings REST API developer docs: https://developer.airthings.com/docs/api-getting-started/index.html
Info about the the Airthings products: https://www.airthings.com/.

Start with the Airthings REST API developer documentation linked in the issue to identify authentication, available sensor metrics, and collection endpoints. Then review Telegraf's existing input-plugin conventions; done means a plugin can fetch and expose the requested Airthings sensor data, with appropriate coverage for the supported API behavior.

Written by the indexing model from the issue text.