Add power switch only if it is available at Home Connect

[Diegorro98]

Breaking change

Some appliances might lose their power entity as the power setting is not reported by the API.

Proposed change

Power switch was always added, and there might be cases where the API does not provide the setting for that appliance, if the setting is not provided is probably that when turning on/off, the API reports an error. This can confuse the user as it already happened in issues like #98537, #109747

Also, the power switch entity description has been moved to a constant so it is not generated every time that a power switch is initialized

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
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• 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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[home-assistant]

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

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