=str #16233 adds ReadAll for FlexiMerge

With one discussion point - what should we expose in java/scala apis? `Array` in both - in scala then we cant pattern match on it, but maybe that's not important anyway hm

Refs #16233
ok cool - yes, `InputHandle` is all over the place, would be lots of `[_]` from what I saw.. Keeping as is then :)