As part of adding the slot-based execution (SBE) engine, we will have multiple implementations of the PlanExecutor interface: one for the "classic" execution engine and another for SBE. This will require us to convert PlanExecutor into a clean interface that can abstract over multiple execution engines (see SERVER-48477). Therefore, it would make sense to have a third implementation of PlanExecutor which is used for executing Pipelines of DocumentSources. This will allow us to get rid of the PipelineProxyStage which currently sits between PlanExecutor and the Pipeline. Using PipelineProxyStage when SBE is enabled is particularly undesirable because it causes the runtime execution plan tree to involve both SBE PlanStages and classic PlanStages.