Add notification via Signal messenger

[pbiering]

• based on notify-by-email script and templates
• requires running Signal messenger client accessable via D-BUS

[sni]

Hi Peter,

i appreciate the work for this PR but it does not really fit into our road map. We could just
merge it, but it probably might result in config merge issues in the notification_commands.cfg.
So i'll close this PR but keep it as an example.
It would also fit well in the show/tell section of the discussions: https://github.com/orgs/ConSol-Monitoring/discussions/categories/show-and-tell

[pbiering]

done: https://github.com/orgs/ConSol-Monitoring/discussions/211