Complete partly implemented type hints for dict on MQTT platforms

[jbouwh]

Proposed change

Complete partly implemented type hints for dict on MQTT platforms:

1. Type the rendered JSON payload for the MQTT update platform.
2. Complete some partly typed dicts used for discovery data

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 #
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• Link to documentation pull request:

Checklist

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[home-assistant]

Hey there @emontnemery, 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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[frenck]

Thanks, @jbouwh 👍

../Frenck