Add ability to 'subscribe' to external WebDAV address books

[meichthys]

Is your feature request related to a problem? Please describe.

Currently the only way to view contacts from external address books is to import the address book into Nextcloud.

Describe the solution you'd like

I'd like to see an option to 'subscribe' to an external WebDAV address book

Describe alternatives you've considered

This would prevent users from having to have duplicate address books - like one on their email provider and one in Nextcloud.

Additional context

Related to: https://github.com/nextcloud/server/issues/252