GH-3923: Add @MessagingGateway @Import support

[artembilan]

Fixes #3923

Currently, the @MessagingGateway interfaces can be scanned or created explicitly as @Bean definition for AnnotationGatewayProxyFactoryBean

• Implement a GatewayProxyBeanDefinitionPostProcessor which is invoked before instantiation attempt on the BeanDefinition
• Verify @Import for @MessagingGateway interface in the GatewayInterfaceTests

[artembilan]

With this InstantiationAwareBeanPostProcessor feature it looks like we don't need a MessagingGatewayRegistrar to be used for a @IntegrationComponentScan: we just simply can rely on a standard AnnotatedBeanDefinition registration against an interface and and delegate to this post-processor to let the AnnotationGatewayProxyFactoryBean to do the trick with annotation attributes processing.
But this is going to be a different story already after merging this one and naming PR.

[chrylis]

Very interesting! Will this functionality make it back to 5.x, or will it be 6.x only? (The re-baseline from Java 11 to Java 17 will be a hassle if required.)

[artembilan]

Well, I'm a bit reluctant to introduce any breaking changes into maintenance versions.
More over this is not a bug, but a new feature: we never advertised an @Import for @MessagingGateway.
As a workaround it is simply enough to add that GatewayProxyInstantiationPostProcessor as a static bean into your own configuration.

[chrylis]

Thanks for this! We'll be stuck on 5.x for a while, but I can continue using the proof-of-concept I have until we overhaul to Java 17.