Fixes https://github.com/spring-projects/spring-integration/issues/2788
* Introduce a `MessageProducerSupport.subscribeToPublisher(Publisher<Message<?>>)`
for components which produces `Flux` for data from their source
* Such a component is auto-stopped when subscription to that `Publisher` is canceled
* Implement a `MongoDbChangeStreamMessageProducer` based on the reactive support for
in Spring Data MongoDb
* Implement a Java DSL for `MongoDbChangeStreamMessageProducer`
* Disable a test for change stream since it requires server of version 4.x started with 'replSet' option
* Add `MongoHeaders` for change stream events
* Change `MessageProducerSupport` to use a `takeWhile((message) -> isRunning())`
instead of storing a `subscription` from a callback
* Document new features
* Remove trailing whitespaces
* Doc Polishing.
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