JIRA: https://jira.spring.io/browse/INT-4100
* Add `throws MessagingException` to the `RequestReplyExchanger` to make it fully messaging contract based
* Add test to the `ServiceActivatorDefaultFrameworkMethodTests` to demonstrate how many info is in the stack trace after `RequestReplyExchanger`
* Document how to be with the `throws` clause in custom gateways
* Fix `boolean` attributes in the `spring-integration-5.0.xsd`
* Fix `AbstractChannelAdapterParser` do not populate endpoint attributes if component is nested
* Fix typo in the `AmqpOutboundChannelAdapterParserTests`
* Increase timeouts in the `MixedDispatcherConfigurationScenarioTests`
Reinstate `auto-startup` to none by default.
Not all components expects `true` by default in their initial state
Also https://jira.spring.io/browse/INT-3432
Document the behavior of the `throws` clause on gateway method
Remove unrelated `bridge` id from the `<chain>`
Add What's new about `RequestReplyExchanger` changes
Doc Polishing.
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