Implement sharing

[Trekky12]

So first thank you for creating a new alternative of a CalDAV/CardDAV server 👍

I recently migrated from nextcloud to baikal to have a more leightweight calendar/contacts server. If I had discovered rustical earlier I think I would have used that instead.

With baikal I really like the implemented sharing feature which has the possibility to show the owners and sharee of a calendar directly in the iOS calendar app. Unfortunately there seems to be no further development and some annoying bugs are not fixed:

• A calendar shared with nobody has an empty sharee: Caldav IOS: Shared with Unknown (null) ? sabre-io/Baikal#1090, proposed PR: Set access to 1 on new calendars to handle share with null sabre-io/dav#1581
• Shared calendar owner is wrong: share calendar -> wrong username sabre-io/dav#924, proposed PR: Fix showing owner of shared calendars on iOS sabre-io/dav#1582

So I noticed that you already thought about implementing the sharing as in draft-pot-webdav-resource-sharing-04 (#96 (comment)).

With this discussion I would like to encourage you in implementing sharing. I think the sharing feature is a big improvement and especially on iOS a little bit more convenient than adding individual principal uris for shared calendars.
Additionally I think it is much easier to create and share calendars directly in the calendar app and users don't need to open the frontend of the CalDAV server.

The implementation details of baikal (sabre/dav) seems to be documented here: https://sabre.io/dav/caldav-sharing/

I would really appreciate if you look into sharing :)