Allow passing extension objects to WorkflowApplication

[fjtirado]

In order to further extent implementation customization capabilities, it will be good to be able to associate arbitrary objects to the application, so it can be later accessed.

For example, default http imple use JaxRs client (itself a factory), but it might be handy for some implementors to replace it. Since we do not want to create a hard dependency between the core (where WorkflowApplication is) and jaxrs or any other rest client, the client object might be added by the implementor when building the application and later retrieved in the task implementation class