* GH-3450: Expose PubSub requireSubscribers option
Fixes https://github.com/spring-projects/spring-integration/issues/3450
* Add `PublishSubscribeChannel` ctors with the `requireSubscribers` option
with a direct delegation to the same option of underlying `BroadcastingDispatcher`
* Add factory methods to the `Channels` & `MessageChannels` to expose
this new `requireSubscribers` option
* Expose an XML `require-subscribers` attribute for the `<publish-subscribe-channel`
* Document this new `requireSubscribers` option
* Introduce a global `spring.integration.channels.error.requireSubscribers` property
for a default `errorChannel`
* Document this property and explain its default `true` in the `error-handling.adoc`
* Remove docs for `spring.integration.postProcessDynamicBeans` since it purpose was
removed since version `5.1` in favor of "always post-process behavior" as it was
always with all the `BeanPostProcessor`s
* * Fix language in docs according review
* * Add more docs about `requireSubscribers` and cross-links between chapters
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