Add 'Last update' timestamp attributes for timed HomeWizard sensors

[DCSBL]

Proposed change

Add 'Last update' attributes for gas and peak demand.

• Gas updates once every hour or 5 minutes, the exact timestamp is available. Users can use the build-in 'Last changed/updated', but it won't change if you don't use gas.
• Peak demand current month shows the highest usage of this month. We got some request to show the real timestamp instead 'Last changed'. I think it is useful for new installations.

Type of change

• [ ] Dependency upgrade
• [ ] Bugfix (non-breaking change which fixes an issue)
• [ ] New integration (thank you!)
• [x] 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 #
• This PR is related to issue:
• Link to documentation pull request:

Checklist

• [x] The code change is tested and works locally.
• [x] Local tests pass. Your PR cannot be merged unless tests pass
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• [x] Tests have been added to verify that the new code works.

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• [ ] Documentation added/updated for www.home-assistant.io

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[gjohansson-ST]

Preference would be that these are their own sensors instead of attributes. Also perhaps not enabled by default.

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