Allow binary sensor template to return state unknown

[epenet]

Breaking change

Previously, binary_sensor template entities could have the state on or off, and in case of YAML definition possibly unavailable. However, compared to other entities, it wasn't able to have the unknown state, which has been added.

So, a binary_sensor template entity can now have the on, off, unavailable, or unknown state.

You might need to adapt your templates, or automations/scripts to take this new unknown state into account.

Note: although other values are still possible, it is recommended to always return one of True/False/None.

Proposed change

As follow-up to #60193, and as alternative to #126909/#128963

Proposal for architecture discussion home-assistant/architecture#1148

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
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• Breaking change (fix/feature causing existing functionality to break)
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• This PR fixes or closes issue: fixes #
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[MartinHjelmare]

I don't believe this is critical for the November release.

[MartinHjelmare]

The architecture discussion isn't approved yet. Draft for now.

[github-actions]

There hasn't been any activity on this pull request recently. This pull request has been automatically marked as stale because of that and will be closed if no further activity occurs within 7 days.
If you are the author of this PR, please leave a comment if you want to keep it open. Also, please rebase your PR onto the latest dev branch to ensure that it's up to date with the latest changes.
Thank you for your contribution!

[Petro31]

Any reason this is still a draft?

[epenet]

Any reason this is still a draft?

Still waiting on arch discussion I think...
Cc @frenck / @MartinHjelmare