Revise the `IntegrationComponentSpec.get()` API to disallow to be called from end-user code

[artembilan]

The IntegrationComponentSpec implements a FactoryBean and we usually don't call its getObject() in the target project.
We rather rely on the Spring Framework dependency injection lifecylce.
The IntegrationComponentSpec.get() manual call may lead the component to the state where it is not fully initialized.
The Java DSL API and the framework by itself is able to deal properly with IntegrationComponentSpec as a bean as it can deal with FactoryBean.

Related issue: #2995