Move KNX keyring validation and storage to helper module

[farmio]

Proposed change

Move KNX keyring validation and storage to helper module.
This shall be used not only by the ConfigFlow, but also by the frontend panel in the future, so I think it makes sense to move it to a dedicated module and refactor the storage method to a generic function.

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• Dependency upgrade
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• This PR fixes or closes issue: fixes #
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[home-assistant]

Hey there @Julius2342, @marvin-w, mind taking a look at this pull request as it has been labeled with an integration (knx) you are listed as a code owner for? Thanks!

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[marvin-w]

Nice! Thanks!