Add switch as entity platform on MQTT subentries

[jbouwh]

Proposed change

Add the MQTT sensor platform to subentry support.
Based on/needed:

• Add initial MQTT subentry support for notify entities #138461 (Main subentry PR with Notify platform)
• Add availability support for MQTT subentries #138673 (Availability)
• Add sensor as entity platform on MQTT subentries #139899 (Sensor)
• Move QoS setting to shared device properties in MQTT device subentries configuration #141369

Follow up PR:
#141345 (Light)

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

Additional information

• This PR fixes or closes issue: fixes #
• This PR is related to issue:
• Link to documentation pull request: TODO
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• Link to frontend pull request:

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[home-assistant]

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

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[elupus]

OT for this change. But this is going to get messy fast if more config flow integrations need to select a device class (and/or units)

[jbouwh]

True, already discussed this with Erik and Joost. But not sure we have an alternative yet.