Wrap MessageGroupProcessor result a collection of payloads into a message in the AbstractCorrelatingMessageHandler

[artembilan]

Right now, the AbstractCorrelatingMessageHandler fails like:

		Assert.isAssignable(Message.class, commonElementType, () ->
				"The expected collection of Messages contains non-Message element: " + commonElementType);

if custom MessageGroupProcessor (which simply can be lambda) returns just a collection of payloads, but not messages.

The DefaultAggregateHeadersFunction can be used in this case to wrap that collection into a single message with grouped headers.
Similar to what happens in the AbstractAggregatingMessageGroupProcessor.

More info in: https://stackoverflow.com/questions/79253871/stop-execution-splitter