module: Signal support

[Quartzic]

Hi,

I'd love to see this project support Signal database backups. These cannot be created through the Signal app, but it is possible to extract the DB from a desktop installation. I would be happy to provide the file formats and a couple rows of my database backups if that would be helpful.

[karlicoss]

Hi! I never used Signal, so sadly don't have much motivation (and especially time!) to do this myself, at least in the nearest future. However, if you attach this demo database, this would definitely be helpful in case someone wants to pick this up later!

[karlicoss]

btw, some inspiration for this can be borrowed from https://github.com/karlicoss/promnesia/blob/master/src/promnesia/sources/signal.py