Inspired by SERVER-97356. We have the ability to return a batch of documents to feed a pipeline, and even to do so with a callback. We should extend this ability to allow a callback function to provide results in a stream. Something like this:

class InputDataStream {
    virtual DocumentSource::GetNextResult getNext() = 0;
};

// allow constructing a DocumentSourceQueue with some implementation of the above.

Conceptually, this might allow us to revert some pieces of SERVER-97356. We could re-implement new queue-like stages as a $queue, and keep all the special cases for $queue. I'm not sure it's worth changing the name back, but I like this design better.