AggregateReduceFunctionsRule should accept arbitrary predicates

AggregateReduceFunctionsRule allows you to specify which functions to reduce, but only allows you to specify by function type. For instance, you can either reduce all SUM functions, or no SUM functions, but not a subset of SUM functions (say, only the SUM functions with distinct keys). Allowing the user to specify an arbitrary predicate would allow us to selectively apply the rule to functions without distinct keys, which matters in Looker due to the fact that distinct keys cause an aggregate to be impossible to roll up, and when that's the case, we'd like to avoid reducing the functions for the purpose of aggregate awareness.