Add date device class

[scop]

Proposed change

Per home-assistant/architecture#610

No usages yet, but I'm planning to add one soonish. The reason I'm not bundling this with that mentioned change is that I'd rather not delay its inclusion in case e.g. implementing support for this device class in the frontend starts to take some time -- in the meanwhile I can use DEVICE_CLASS_TIMESTAMP sufficiently.

Child PR's:

• Add date device class frontend#9983
• Add date device class home-assistant.io#19249
• Add date device class developers.home-assistant#1053

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)
• 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: Support for plain date device class architecture#610
• Link to documentation pull request:

Checklist

• 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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[MartinHjelmare]

Please see this PR and all its linked PRs for a template of what needs to be included when adding a sensor device class:

#55050

[scop]

Thanks. Expanded this one some, and added child PR's, links in OP. Looking at how timestamp is implemented, I suppose like it, this'll need fewer modifications than the voc adddition.

[MartinHjelmare]

Yeah, the lack of a unit of measurement for the device class will decrease the needed changes.

[MartinHjelmare]

Thanks!