* GH-2695: Add proxy option to @EnablePublisher
Fixesspring-projects/spring-integration#2695
To configure a `proxyTargetClass=true` we need declare a
`PublisherAnnotationBeanPostProcessor` bean manually, but that may cause
a confuse when `@EnablePublisher` is still present.
So, target service is proxied twice
* Expose `proxyTargetClass` and `order` into the `@EnablePublisher`
and `<enable-publisher>`
* Refactor `PublisherAnnotationBeanPostProcessor` to extend an
`AbstractBeanFactoryAwareAdvisingPostProcessor` to avoid AOP boilerplate
code altogether
* Add assertion into the `PublisherAnnotationBeanPostProcessor` to be
sure that only one of its instance is present in the application context
* * Polishing error message and Docs
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