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_prefix: http://static.site.com/

Will transform var/site/file.png to http://static.site.com/var/site/file.png.