Add WiFi Signal Strength as sensor

[silentgecko]

Just add a sensor for wifi signal strength. Did this in my own lilygo higrow devices ;)

[bruvv]

Thanks for this could be useful for people! After build is ready I will merge it. Build failed, @silentgecko can you fix it? Then I will merge it. Should be an easy fix ;)

[silentgecko]

sorry, it's a sensor, not a text_sensor - totally missed that 😄 i added an icon, too

[bruvv]

Thanks! merged