Add device class to ZHA Xiaomi plug "consumer connected" sensor

[TheJulianJES]

Proposed change

This adds the plug device class for binary sensors that specify if a plug is plugged into a Xiaomi/Aqara EU smart plug.
Small follow-up to: #88194

Also confirmed that this doesn't create a new entity, but that it updates the existing one like expected.

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

Additional information

• This PR fixes or closes issue: fixes #
• This PR is related to issue:
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[home-assistant]

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

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