The FunctionInspector needs to be able to distinguish between a
function of Flux<Foo> and a function of Flux<Message<Foo>>. Then
it can do the conversion a level below.
Only supports Message->Message or POJO->POJO (no mixtures), so there
is only one new method in FunctionInspector.
No support in the web endpoints yet. But it's probably not so hard
to add.
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