Support LWT messages when using TASMESH

[pipe01]

Description:

This PR enables the TASMESH broker to detect when nodes come online and offline through the use of heartbeat packets. When a change is detected, the corresponding LWT Online or Offline message is sent through MQTT.

Related issue (if applicable): fixes #20369 and #20375

Checklist:

• The pull request is done against the latest development branch
• Only relevant files were touched
• Only one feature/fix was added per PR and the code change compiles without warnings
• The code change is tested and works with Tasmota core ESP8266 V.2.7.4.9
• The code change is tested and works with Tasmota core ESP32 V.2.0.14
• I accept the CLA.

NOTE: The code change must pass CI tests. Your PR cannot be merged unless tests pass

[s-hadinger]

Please change the defines

[github-actions]

This PR has been automatically marked as stale because it hasn't any activity in last few weeks. It will be closed if no further activity occurs. Thank you for your contributions.

[Jason2866]

@Staars fine for you?

[Staars]

No complaints from my side.
We had the topic of LWT on our radar relatively early on, but have not yet started any concrete implementations. These are always best created based on concrete problems and solutions. I assume the PR works as intended and has been tested for some time.

I remember thinking relatively early on about the concept of different nodes, which could be "always on" or "uncertainly accessible". The former should send a guaranteed heartbeat, where the absence would indicate an error.
The latter would only report when they wake up from a deep sleep, for example, and would not take part in forwarding data packets. In order to control this remotely, a functioning LWT would be necessary. However, due to a lack of concrete applications, I didn't pursue the concept any further.