INT-651: Add `Iterator` support for `Splitter`

JIRA: https://jira.spring.io/browse/INT-651
Something went wrong here - it can still be a collection; perhaps change to something like...

A collection or an array of Messages, or an Iteratable (or Iterator) that iterates over Messages

We also should indicate the sequence size is 0 when using iterators, so the default aggregator won't work.

We should also explain why you might choose to use an iterator - it avoids building an entire collection in memory before splitting.

It's quite a specialized use case, we're not really "splitting" the inbound message itself, just that the inbound message contains metadata to build an iterator of some kind.