JIRA: https://jira.spring.io/browse/INT-4134
* Introduce `AnnotationGatewayProxyFactoryBean` to parse `@MessagingGateway` annotation directly in the bean.
Useful for Java (`@Bean`) configuration variant
* Use `AnnotationGatewayProxyFactoryBean` for the newly introduced `IntegrationFlows.from(Class<?>)`
* Override `requestChannel` in the result gateways to the auto-created `DirectChannel` in the current `IntegrationFlowDefinition`
* Register `AnnotationGatewayProxyFactoryBean` from the `IntegrationFlows.from(Class<?>)` as a bean with the name like: `IntegrationFlow` bean name plus `.gateway` suffix
* Merge `GatewayCompletableFutureProxyFactoryBean` to `GatewayProxyFactoryBean`, since we are based on the Java 8 already anyway
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