resource provider should be two-staged

[lueck]

Resource providers should be a two-staged provider:

1. let the first stage provider make a second stage provider from the request's provider-related parameters
2. call the second-stage providers openStream() etc. methods and pass the input stream along with second stage the stream to the transformation

Only this way, we can process subsequent requests by the transformer plugin when it comes to more complicated resource providers that download and cache a repository of files.