=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
the "decide" part of it yes, the handle it no. I'll need to carry some information in `ReadAll` in order to know if this should emit an array (javadsl) or List (scaladsl).