Add a Signal App bridge

Is your enhancement request related to a problem? Please describe.
I want to establish a trustworthy relationship with my customer, especially the ones that care for security and dislike other solutions

Describe the solution you'd like
Let's build a signal bridge with and their reference implementation for a a cli, that could be extended to call the api.

Describe alternatives you've considered
Use a telegram-bot, but it's also not so secure.