Add support for energy consumption rate entity

[cascer1]

Currently, the card only supports a number value for consumption rate.

I think it would be useful to be able to set an entity instead of a number. For me, this is useful because my energy consumption rate changes hourly and I would set an entity tracking the average value to this.

[wawyed]

This would very useful as I'm also on a tariff that supports multiple rates depending on the time of the day