Fixes: https://github.com/spring-projects/spring-integration/issues/9754
Right now a correlation handler can discard messages in the expired group one by one.
In some scenarios it would be useful to have single message in discard for the whole group.
* Expose `discardIndividuallyOnExpiry` for the `AbstractCorrelatingMessageHandler`,
and `AggregatorFactoryBean`, and respective `CorrelationHandlerSpec` for DSL.
This new option takes action only if a `discardChannel` is provided,
and `sendPartialResultOnExpiry` is not set to `true`.
When `discardIndividuallyOnExpiry` is false, the messages in the expired group are packed
into a list for payload of a discarding single message.
* Test and document the new feature
ad61cef7fc