Add combination.expression to ScoreFusionSpec (and remove the old "score" field on the spec) and implement corresponding behavior in document_source_score_fusion.

More info about combination.expression here. 

Per design review discussions, should appropriately handle cases where pipeline name is used for a variable in combination.expression (ex: check whether variable is in scope).

Add a test to handle $let in the expression which can be used to duplicate a variable but the duplicated variable should follow natural scoping behavior and ultimately not cause confusion.