Add possibility to switch relay state via rules

[NerdyProjects]

I (want to) use the relay to tell an external heating system controller to turn on the pumps / set the mixing valves and to not rely on another computer, I would like that to happen via rules on heishamon.

[Roelaynd]

I concur, I would like to have control over the relays via the rules as well for setting a valve instead of having to rely on Home Assistant.

(Great job thus far btw, Heishamon can't be praised enough)

[blb4github]

This would be a great feature!!

[Djongambo]

I would also highly appreciate that to control an additional external pump. Need to switch that pump off when heatpump goes into defrost mode.