Due to the introduction of Expression into MatchExpression, we now use the ExpressionContext inside of MatchExpression. But ExpressionContext has members such as AggregationRequest parameters and an owned CollatorInterface that do not make sense outside of aggregation. We would like ExpressionContext to only contain members relevant to Expression (opCtx, variables, variablesParseState, and _collator) and make a new AggregationContext to hold an ExpressionContext and all the other state relevant to aggregation. AggregationContext will be used inside of aggregation, and an Expression should only hold an ExpressionContext, which makes sense to use outside of aggregation.