=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
sounds interesting - and termination? I expected that we want it to complete if at least one of the inputs completes/is-cancelled (reworking it to work like that as we type).