Introduce a way to get "child" elements of the `Resource` abstraction

[geoand]

I encountered a case where I need to replace something that contains Path with something that will use Resourcebut I really need to know to be able to get the contents of the directory - if it is one.

[dmlloyd]

The methods Resource#isDirectory()/Resource#openDirectoryStream() should work for this use case. Could you try it?

[geoand]

That does the trick, thanks!