Added `ntfy.sh` support to Borgmatic

As a “homelab” Borgmatic user, I found the available options for push notifications a little limiting. Having become aware of ntfy.sh which offers both a cloud-hosted and self-hosted version, this seemed a good fit into Borgmatic to allow custom push notifications to be sent. So I set about creating a pull request for the Borgmatic project to implement this.

This PR followed all the project guidelines, including writing basic unit tests, caters for privacy-conscious self-hosted Ntfy users just as easily as the cloud-hosted version, and comes with full documentation

See the pull request with the code which was merged into borgmatic 1.6.3