Use more descriptive water heater states

[cryptk]

When looking into #82 I noticed that currently the water heater gives a state of either "on" or "off" while we should only use the states defined within https://developers.home-assistant.io/docs/core/entity/water-heater#states

The overkiz atlantic_pass_apc_dhw integration in HA core has some nice examples of returning the correct state based on the status of a few different parameters that can be used for reference to allow returning eco/performance/gas/etc in multi heater setups with solar and electric/gas/heat pump/