Add and unit-test a dynamic stage mapping method with provisionary signature

namespace mongo::exec::agg {
  intrusive_ptr<Stage> buildStage(const intrusive_ptr<DocumentSource>&);
}

The method should use a global map (e.g., attached to the service context) containing dynamically-added mapping functions for each document source type ID. Initially, this map should be populated with dummy/epsilon mapping functions simply casting intrusive_ptr<DocmentSource> to intrusive_ptr<agg::Stage> and returning a copy of the ref-counted pointer.

Since we already have document_source_visitor_registry.h which already 'hangs' on the service context as a decorator, we should evaluate if we can re-use it for this purpose.