Add reauth source constant for config entries

[ctalkington]

Breaking change

Proposed change

Frontend already uses this "magic" string to display warning to user and multiple integrations implement reauth with this source.

ref home-assistant/frontend#6808

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• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New integration (thank you!)
• New feature (which adds functionality to an existing integration)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Example entry for configuration.yaml:

# Example configuration.yaml

Additional information

• This PR fixes or closes issue: fixes #
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[MartinHjelmare]

I think this is ok for now until we've fleshed out how this should work in the future.

[bdraco]

frontend already supports it, seems fine to me