Add combination.method to ScoreFusionSpec and implement corresponding behavior in document_source_score_fusion.cpp

More info about combination.method here. 

After the initial desugaring ticket, the scores are combined by implicitly by summing together the input scores. In this ticket we want to add 'combination.method' field to the ScoreFusionSpec, and then alter the desugared stages based on the option request. Valid options should be: "sum", and "avg"

For this ticket - ignore implementing "combination.expression" - that will be handled in the follow on to this