API: Dispatch mechanism for EA reductions
https://github.com/pandas-dev/pandas/pull/33538 implemented sum as a standalone method for IntegerArray which somewhat duplicates the already-existing _reduce path taken by DataFrame / Series for certain reductions (see https://github.com/pandas-dev/pandas/pull/33538#pullrequestreview-393402534). Likely it makes sense to implement these at the IntegerArray / BooleanArray / etc. level using that same approach, e.g., something like defining each reduction to simply call _reduce with the appropriate operation name instead of copy / pasting the logic into each method.