DFS: Added UrlDecorator

A service that decorates an URL (var/site/file.png), for instance
with a given prefix.

Can be configured for flysystem binarydata handler:

ez_dfs:
  nfs:
    binarydata:
      flysystem:
        adapter: nfs
        url_decorator:
          prefix:
            prefix: http://static.site.com/

Will transform var/site/file.png to http://static.site.com/var/site/file.png.
DFS: Fixed ApiLoader test
DFS: Made UrlDecorator optional
Merge branch 'EZP-23328-dfs_io-url_decorator' into EZP-23328-dfs_io
IO: 'Fixed' handling of image uri in XML