Planner Updates

Update planner to push down $unwinds in eligible prefixes.
 

Stage Builder Updates

Update stage builder to build pushed-down $unwind stages.

 
SBE Plan Cache

Encode pushed down $unwind stages in the plan cache key (canonical_query_encoder.cpp encodePipeline()).'

 
Update Existing Jstests

Many tests examine explain output, some instances of which will change when $unwind is pushed down so will need alternate expected results based on featureFlagSbeFull during development. The old expected results will be removed along with featureFlagSbeFull when the feature is complete.