Currently, we pass the resolved and desugared subpipeline for $unionWith/$graphLookup/$lookup through StageParams as a `boost::optional<OwnedLiteParsedPipeline>`. It would be ideal if StageParams did not have to rely on LPP or DocumentSource at all and was its own fully separate library.

Once stages are fully migrated to StageParams, we should be able to conveniently just store `boost::optional<StageParamsPipeline>`, rather than LPDS, since StageParams is the lightweight version of describing "what is a stage" without having to go back and forth to BSON.