Make returned context pluggable via an URL pattern on REST API

The extension point should contribute classes that knows how to return a HashMap<String,Object> and that is given a documentModel and a string (the name of the context to give). We need :

• a service that expose a registry of context contributors. Contributors will react upon given headers and the contextual document.

• to use this service in the DocumentModelWriter in order to fill the contextParam

The goal is to be able to return additional resources within a document, if the client put a specific header in the request and have this behavior pluggable.

A sample use case of this feature could be to have a header to return children of the document