=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
could we keep this 2-way zip sample as it was? illustrates the state transitions nicely
we could add a 3-way zip that uses ReadAll
I think we can use some of these samples in the docs.