DSL: Add support for IntegrationFlow Lambda

To simplify a bit the `direct` integration flow definition, the `IntegrationFlow` Lambda is introduced.

* Extract `IntegrationFlow` functional interface with `define(IntegrationFlowDefinition<?> flow)` method.
* Rename existing `IntegrationFlow` class to the `StandardIntegrationFlow` and move it to the inner class within `IntegrationFlowBuilder`
* Extract `IntegrationFlowDefinition` superclass for the `IntegrationFlowBuilder` to be used within `IntegrationFlow` Lambda
to hide `get()` method from end-users.
* Change `IntegrationFlowBeanPostProcessor` for a new logic around `IntegrationFlow` Lambda
* Rename `MessagingProducerSpec` to the `MessageProducerSpec`
Yes, it doesn't matter if it is Lambda or direct `IntegrationFlow` implementation. I have just not found the better name for the method :smile: . Your thoughts ?