Add a last_seen sensor

[magicus]

I have a wifi stability issue with the range extender to which my Heru is connected. From time to time, I lose connection and need to reset the range extender.

It would be great if the Heru integration could publish a sensor (like <heru-device>_last_seen) with a timestamp of the last known sign of life from the Heru (i.e., the last time the integration received some value).

(Right now my solution is to manually discover that it has lost connection again, and then manually take action to restore connectivity.)