Add serial_number to device registry entries

[emontnemery]

Proposed change

Add serial_number to device registry entries

The serial number can be displayed in the device card in frontend

Type of change

• [ ] Dependency upgrade
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• [ ] Code quality improvements to existing code or addition of tests

Additional information

• This PR fixes or closes issue: fixes #
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Checklist

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[probot-home-assistant[bot]]

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[bdraco]

Related arch discussion

https://github.com/home-assistant/architecture/discussions/717

[emontnemery]

We agreed this was OK. The related architecture discussion can be solved by frontend simply picking MAC(s) from the device entry and displaying that.

[frenck]

The related architecture discussion can be solved by frontend simply picking MAC(s) from the device entry and displaying that.

I'm confused by this comment @emontnemery, so your are saying MAC addresses could be just picked up by the frontend from the identifiers...

Why isn't serial in the identifiers as well in that case?

That comment, just made this PR invalid IMHO.