Add sensor for grid frequency without power switch

[weltenwort]

📝 Summary

This adds a sensor for the grid frequency that also works for systems without a power switch as requested and implemented by @k3mpaxl.

closes #134

[github-actions]

🎉 HACS repository validator action summary 🎉
✅ The repository is not archived
✅ manifest.json file exist
✅ All required keys are present in manifest.json
✅ hacs.json has the 'name' key set
✅ The repository has a description
✅ rct_power is added to https://github.com/home-assistant/brands NICE!
✅ The repository has topics
✅ The repository has issues enabled
✅ This day ends with an 'y' (Thursday)
✅ info.md exists
✅ Requirements validation
✅ HACS load check