Functional gateway bean definitions

[artembilan]

• Rework MessagingGatewayRegistrar to parse messaging gateway annotation
  an <gateway> XML using a supplier variant for bean definition.
  Such a feature is required by Spring Native - otherwise we would need to
  register reflection info for to many internal Spring Integration classes
• Such a change should benefit from regular JDK perspective, too -
  we don't do reflection for this kind of bean registrations

[artembilan]

NOTE: I'll let to know when it is ready for merging.
Didn't have a chance to test it from Spring Native perspective, yet.

[artembilan]

OK. As long as I see this one in native logs:

Caused by: com.oracle.svm.core.jdk.UnsupportedFeatureError: Proxy class defined by interfaces [interface com.example.integration.ControlBusGateway, interface org.springframework.aop.SpringProxy, interface org.springframework.aop.framework.Advised, interface org.springframework.core.DecoratingProxy] not found. 

I can tell that GatewayProxyFactoryBean is created properly.
The Proxy configuration concern is out of scope of the work done in this PR and has to be addressed on Spring Native side.

[artembilan]

See Spring Native PR for more info: spring-attic/spring-native#894