FilesHome now also returns DAV properties

The files home node must also return DAV properties like etag,
permissions, etc for the clients to work like they did with the old
endpoint.

This fix makes FilesHome extend the Sabre Directory class, this makes
the FilesPlugin and other plugins recognize it as a directory and will
retrieve the matching properties when applicable.

Downstream of https://github.com/owncloud/core/pull/26066



Signed-off-by: Lukas Reschke <lukas@statuscode.ch>