Add state-less statistics entity for use with Energy Dashboard

[steinmn]

Inspired by https://www.home-assistant.io/integrations/tibber/

This allows for properly backdating hourly power consumption which solves the offset issue seen in #162 and #103

Will require implementing the chargehistory-api, and probably pagination as well.

[sveinse]

This is the code for Tibber: https://github.com/home-assistant/core/tree/dev/homeassistant/components/tibber

[steinmn]

More specifically, the _insert_statistics function is what we need to learn from: https://github.com/home-assistant/core/blob/dev/homeassistant/components/tibber/coordinator.py#L70

[sveinse]

Will require implementing the chargehistory-api, and probably pagination as well.

Yes and good. I was worried there for a second that we needed to make the integration hold history. Let's cross our fingers that Zaptec portal contains all information we need to implement this.