* GH-3155: Add support for Java DSL extensions
Fixes https://github.com/spring-projects/spring-integration/issues/3155
Provide an `IntegrationFlowExtension` for possible custom EI-operators
in the target project use-cases.
* * Move `IntegrationFlowExtension` tests ot its own test class
* Make all the `IntegrationComponentSpec` ctors as `protected` for possible custom extensions
* Make some `BaseIntegrationFlowDefinition` methods and properties as `protected` to get them
access from the `IntegrationFlowExtension` implementations
* Document the feature
* * Fix language and typos in docs
* * Add `protected` to one more `GatewayEndpointSpec` ctor
* Add JavaDocs to `GatewayEndpointSpec` methods
* * Add `protected` to one more `JmsPollableMessageChannelSpec` ctor
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