Standardize radon units & add converter

[Kirchoff]

Breaking change

Proposed change

I am proposing that the few existing components that contain radon sensors be standardized. This includes airq, airthings, and airthings_ble. Each of these components handle units, state, and types differently. This PR aims to change that by introducing RADON as a SensorDeviceClass, as well as adding a standard unit converter, RadonConverter. This will allow users to set their own units, depending on their preference, instead of relying on the component to do so.

"Bq/m^3" is the standard SI units for measuring radon radioactivity concentration, "pCi/L" is the imperial equivalent used in the United States.

Please provide feedback if things should be implemented differently, and especially if/how new tests should be added for the new SensorDeviceClass.RADON and RadonConverter types.

Thank you.

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New integration (thank you!)
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• Deprecation (breaking change to happen in the future)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

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• This PR fixes or closes issue: fixes #
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[Kirchoff]

@bdraco - Thank you, discussion started.

[MartinHjelmare]

Please open a new PR when the architecture discussion is approved. The initial PR here seems to have included unrelated commits that pinged a lot of contributors. If we keep this PR alive they will get spammed for every update to the PR. Thanks!