Change the explain format for sharded aggregations to accomodate $exchange

In a world where $exchange could be inserted theoretically anywhere in a sharded aggregation, it won't make sense to always present the "splitPipeline" in aggregation explain as a "shardsPart" and "mergerPart".

One potential format is to break up the pipeline into "bubbles", where each one is separated by $exchange. At the exchange points, we could expose detail on which producers are feeding each consumer, to give a sense of how the routing is occurring.