* GH-3192: pub-sub DSL for broker-backed channels
Fixes https://github.com/spring-projects/spring-integration/issues/3192
* Introduce a `BroadcastCapableChannel` abstract to indicate those `SubscribableChannel`
implementations which can provide a pub-sub functionality
* Implement a `BroadcastCapableChannel` in broker-baked channels with pub-sub option
* Introduce a `BaseIntegrationFlowDefinition.publishSubscribeChannel()` based
on the `BroadcastCapableChannel` and `BroadcastPublishSubscribeSpec` to let to
configure sub-flow subscribers in fluent manner
* * Add some JavaDocs and document new feature
* * Show the channel bean definition in the doc
* Fix typo
87c8e47a88