feat: Export the main and create __main__.py

[Its-Just-Nans]

Fixes/Implement: Export the main and create __main__.py

Description:

Export the main and create __main__.py

This file allows to direclty execute the main

python -m pip install mqtt-exporter
python -m mqtt_exporter

[Its-Just-Nans]

Fixed the CI

[kpetremann]

thanks!

[kpetremann]

note: you could already run the project via : python -m mqtt_exporter.main

[Its-Just-Nans]

Yes, but having __main__.py is a little bit cleaner ^^