Revise splitterFunction signatunre into Function<Flux<Message<?>, Flux<Message<?>>>

[artembilan]

When we have a composition like this:

spring.cloud.function.definition = fileSupplier|splitterFunction

Then final "function" signature is like this Supplier<Flux<Message<List<Message<?>>>>>.
And that is exactly what we don't expected from the splitter in the end of the composition.
While Spring Cloud Stream supports de-batching, it works for a List output only if function is bound by itself.
In case of composition we got just a Supplier.

Changing signature for the splitterFunction to reactive types would make it working even with a Supplier composition.
The workaround is like this extra function in the end of the composition:

	@Bean
	Function<Flux<Message<List<Message<?>>>>, Flux<Message<?>>> flattenFunction() {
		return messageFlux -> messageFlux.map(Message::getPayload).flatMapIterable(Function.identity());
	}