Add source entity attributes to power sensor

[bramstroker]

No description provided.

[github-actions]

🎉 HACS repository validator action summary 🎉
✅ The repository is not archived
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✅ All required keys are present in manifest.json
✅ hacs.json has the 'name' key set
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✅ powercalc is added to https://github.com/home-assistant/brands NICE!
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✅ This day ends with an 'y' (Saturday)
✅ README.md exists
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