Sessions: add price/co2 savings

[naltatis]

fixes https://github.com/evcc-io/evcc/issues/11160

• 💰 calculate and persist co2/price savings per session
• 📊 use average of all currently visible rates as base level (reference)

TODO

• [ ] fix and add tests
• [ ] decide on how to combine old savings (global calculation, now = reference) with the new more accurate logic

[naltatis]

Idea to reconstruct old savings: https://github.com/evcc-io/evcc/issues/11160#issuecomment-2333367132