DSL: Add `File.` Namespace Builder Factory

* Introduce `ComponentsRegistration` *marker* to extract internal components from the `IntegrationComponentSpec` to be registered as bean in the application context
* Add `ConsumerEndpointSpec#order(int order)` for the `order` of target `AbstractMessageHandler`
* Fix `AggregatorSpec` to use `DefaultAggregatingMessageGroupProcessor` by default
* Fix `IntegrationFlowBeanPostProcessor#generateBeanName` to check if `instance instanceof NamedComponent` and its `beanName` has been configured
using `IntegrationComponentSpec#id`
Merge branch 'master' into integration-4.0
Reformatted gradle file inline with the existing standard