make \OC\Files\Node\Node implement the FileInfo interface

Instead of extending I opted for composition which allows lazy loading the fileinfo.

cc @DeepDiver1975 @PVince81 
The implementation is now in `Node` instead of `File` (https://github.com/owncloud/core/pull/12616/files#diff-65e9392078bf80ca2a408a8f8cd1fc65R271)