This is an idea for a simplification in SBE stagebuilders. Currently we have struct PlanStageMetadataSlots with a handful of fields for some of the document metadata fields. In PlanExecutorSBE::initializeAccessors we handle most of them the same way, except for sortKeySlot which has a special case I'm not familiar with.

Could we replace this with a single `map< MetaType, SlotId >`, and handle all meta fields the same way? This would make SBE unaware of the specific meanings of the different meta fields, which seems like a good thing for modularity: SBE provides low level execution, and higher layers can put whatever data they want into it.

I noticed this while reviewing SERVER-99589.