Add support for custom inner pipeline/expressions for payload.

This should use the Pipeline:parse APIs. And only support the project, addFields, and replaceRoot operators.

example starting point: 
auto pipeline = Pipeline::parse(... user's payload pipeline ... )
.. something to feed the pipeline the input documents from the StreamDataMsg .. we might already have some utilities for this ..
Document result = pipeline.getNext()
.. assert only one result is returned ..
 

In the planner call: 

I think you can just call Pipeline::parse , then validate the resulting DocumentSource instances are for $project, $addFields, or $replaceRoot.