Expand AggFormula and RollingFormula to allow list of formula per operator (vs. single formula)

[lbooker42]

Recent PR (#6206 and #6143) changed agg and update_by formula methods to accept multiple columns as input but limited the operators to a single input formula (e.g. agg.formula(formula="out_b=min(X)")). It would be useful to specify multiple formula as a list and create multiple output column with a single call.

The new syntax would be agg.formula(formula=["out_b=min(X)", "out_c=sum(x) + min(y)"]) allowing the user to easily specify either a single formula or a list.