Add extension points for notification providers and add Ntfy as notification provider

[sdebruyn]

Let me know what you think. This builds upon #1007

[AndreyNikiforov]

let's close on smtp notification refactoring first.

My first reaction to adding custom/third party notification channel: no. There is downstream project that specializes on notifications to different providers. However, adding generalized extension point for notifications (e.g. webhooks) might be valuable - better discuss before coding though.