Support CLASS property when sharing objects via CalDAV

[element-code]

RFC 3.8.1.3: Classification / https://icalendar.org/iCalendar-RFC-5545/3-8-1-3-classification.html Relates to https://github.com/sabre-io/dav/issues/760

When having shared calendar instances, it would be very useful if sharees can't read some events. The current situation misleads the user from the UX perspective, as their client lets them set an event or to do as private/confidential, but other users still can see it.

Owncloud has implemented this with https://github.com/owncloud/core/pull/24080