Add sensor 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)

Follow up PR's:

• Move QoS setting to shared device properties in MQTT device subentries configuration #141369 (QoS option to device settings)
• Add switch as entity platform on MQTT subentries #140658 (Switch)
• Add light as entity platform on MQTT subentries #141345 (Light)

Needs: home-assistant/frontend#24733

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:
• Link to documentation pull request: Add MQTT sensor subentry support home-assistant.io#37981
• Link to developer documentation pull request:
• Link to frontend pull request:

Checklist

• [x ] The code change is tested and works locally.
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• Tests have been added to verify that the new code works.

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• Documentation added/updated for www.home-assistant.io

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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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[jbouwh]

Frontend PR was merged

[emontnemery]

It's not easy to understand what the False does.

I'd suggest to make the PlatformField kwarg-only (that can happen in a separate PR though)

[jbouwh]

I'll leave this one for a future PR