Fix CORS registration for IntGraphController

We fail with `NoClassDefFoundError` when we use `@EnableIntegrationGraphController`
in WebFlux env without Spring MVC.
Another issue that we don't register CORS for WebFlux

* Extract top-level package protected classes for MVC and WebFlux to register CORS
in the appropriate environment according classpath
* Adjust `HttpIntegrationConfigurationInitializer` and `WebFluxIntegrationConfigurationInitializer`
for native compatibility
* Add `@Indexed` for `@MessagingGateway` for indexer support in Spring Boot and Spring Native

**Cherry-pick to `5.4.x` & `5.3.x`**

spring-integration-core/src/main/java/org/springframework/integration/annotation/MessagingGateway.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2014-2019 the original author or authors.
+ * Copyright 2014-2021 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -22,6 +22,8 @@ import java.lang.annotation.Retention;
 import java.lang.annotation.RetentionPolicy;
 import java.lang.annotation.Target;
 
+import org.springframework.stereotype.Indexed;
+
 /**
  * A stereotype annotation to provide an Integration Messaging Gateway Proxy
  * ({@code <gateway/>}) as an abstraction over the messaging API. The target
@@ -44,6 +46,7 @@ import java.lang.annotation.Target;
 @Target(ElementType.TYPE)
 @Retention(RetentionPolicy.RUNTIME)
 @Documented
+@Indexed
 public @interface MessagingGateway {
 
 	/**