Make flexit bacnet switch more generic and prepare for more switches

[lellky]

Breaking change

Proposed change

I realised that the code wasn't generic, so I added turn_on_fn and turn_off_fn to the entity description.

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New integration (thank you!)
• New feature (which adds functionality to an existing integration)
• 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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[home-assistant]

Hey there @piotrbulinski, mind taking a look at this pull request as it has been labeled with an integration (flexit_bacnet) you are listed as a code owner for? Thanks!

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[Moustachauve]

This LGTM :)