491af8e42c
Allow the backend to provide a value for the `getcontentlength` property as described in [RFC 2518 section 13.4][1]. The implementation treats is as optional, allthough it is a required property per RFC. Most clients do perfectly fine without it, though. Properly setting this in the backend makes the CalDAV collection listable with clients that do require it, e.g. cadaver. [1]: https://datatracker.ietf.org/doc/html/rfc2518#section-13.4
