Add support for Nextcloud

[antitoine]

It would be quite possible to add a Nextcloud module using the WebDav API: https://docs.nextcloud.com/server/stable/developer_manual/client_apis/WebDAV/search.html

[antitoine]

Webdav doesn't support offset, and a pull request (nextcloud/server#10880) is pending for this subject for a while. We could instead use Nextcloud's internal search API, but it's not necessarily that simple because each request is controlled with a token (CSRF).

[antitoine]

The best solution is to provide a new Nextcloud App that expose the native search API for Quest: https://github.com/antitoine/Quest-nextcloud