Add apprise as notification channel

[TheFehr]

Add https://github.com/caronc/apprise-api as another notification channel.

Origin: https://github.com/eikek/docspell/issues/1293#issuecomment-1014467074

[TheFehr]

Quick question did you mean by proprietary/closed services the receiving service of the notification or the tool/library I shared?

Maybe to explain my use case: I have home assistant running and I try to route all important notifications through there to have a central place and to (if it makes sense) react in some way with connected devices to notifications.

[eikek]

I have no problems with apprise (I adopted the title to add this specific one and added urls, otherwise it's hard to know what to add :))! Speaking in general, adding new notification apis is good and the code should be prepared for that (I hope). Also in general - I'm in favor for services that are also self-hosted and open. I might be ok with PRs, but I won't work on those myself most probably.

[TheFehr]

I totaly understand that you do not want to support all those many services. That is exactly what apprise wanted to solve as far as I could tell. And for those who want to send events from docspell to a proprietary/closed service apprise does support from the list a saw quite a handfull of partly/fully closed/proprietary services.

And yeah my naming was a bit generic.