=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
It is not clear to me from reading this how the output will look if I have cancelled/completed inputs. Will there be holes in the List? How do I know what elements map to what inputs?