* GH-3903: Improve AOT for gateway proxy beans
Fixes https://github.com/spring-projects/spring-integration/issues/3903
* Fix `MessagingGatewayRegistrar` to set a `targetType` on the bean definition
instead of `FactoryBean.OBJECT_TYPE_ATTRIBUTE`
* Rework `GatewayProxyBeanRegistrationAotProcessor` to the
`GatewayProxyInitializationAotProcessor implements BeanFactoryInitializationAotProcessor`
to avoid custom code generation
* Remove tests which rely on the `FactoryBean.OBJECT_TYPE_ATTRIBUTE`
* * Add `setBeanClass(GatewayProxyFactoryBean.class)` to satisfy Spring container expectations
* * Rework `GatewayProxyInitializationAotProcessor` to deal with generics
we are exposing now on the `ProxyFactoryBean`
0d8e286572