Add CalDAV support
Summary:
Notes:
- Add a webdavcommon folder for WebDAV generic resource code
- Move davresource to carddaveresource and make it use the WebDAV code
- For now it tests the CalDAV resource directly on KolabNow (to be changed)
- Only synchronization, not adding / changing / removing WebDAV collections or items (to be implemented)
- Only events are currently supported (todo, freebusy, etc. are to be implemented but should be straightforward)
Fixes T8224
Reviewers: cmollekopf
Tags: Sink
Maniphest Tasks: T8224
Differential Revision: https://phabricator.kde.org/D11741