Create a new header src/mongo/db/exec/expression/evaluate.h and define a namespace mongo::exec::expression with a set of functions

Value evaluate(const ExpressionFromAccumulator& expr, const Document& root, Variables* variables)

one for each

• ExpressionFromAccumulator<AccumulatorAvg>
• ExpressionFromAccumulator<AccumulatorMax>
• ExpressionFromAccumulator<AccumulatorMin>
• ExpressionFromAccumulator<AccumulatorStdDevPop>
• ExpressionFromAccumulator<AccumulatorStdDevSamp>
• ExpressionFromAccumulator<AccumulatorSum>
• ExpressionFromAccumulator<AccumulatorMergeObjects>
• ExpressionFromAccumulatorN<AccumulatorMaxN>
• ExpressionFromAccumulatorN<AccumulatorMinN>
• ExpressionFromAccumulatorN<AccumulatorFirstN>
• ExpressionFromAccumulatorN<AccumulatorLastN>
• ExpressionFromAccumulatorQuantile<AccumulatorMedian>
• ExpressionFromAccumulatorQuantile<AccumulatorPercentile>

The body of the functions will be in a src/mongo/db/exec/expression/evaluate_accumulator.cpp and will be taken from the evaluate() method, and the evaluate() method will be changed so that it calls the new method.