Fire trigger on Tasmesh broker when a node disconnects

[pipe01]

When using Tasmesh, nodes won't send any MQTT Online or Offline messages, meaning that tools like Home Assistant won't be able to use these nodes as they will permanently appear offline. Sending the Online message is easy enough through the use of a rule on the node, e.g. ON system#init DO Backlog MeshNode 00:11:22:33:44:55; Publish tele/tasmota_XXXXX/LWT Online ENDON, however sending the Offline message is currently impossible: the node won't be able to send it as it's now powered off, and the broker never realizes that the node is gone and doesn't fire any triggers.

The best solution would be to somehow implement LWT messages on the broker, but in the meantime a nice workaround would be for the broker to detect when a node has gone offline and print a message which can then be used as a trigger to run a Publish command with the LWT Offline message.