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`**
# Conflicts:
# spring-integration-http/src/main/java/org/springframework/integration/http/config/IntegrationGraphControllerRegistrar.java
# Conflicts:
# spring-integration-http/src/main/java/org/springframework/integration/http/config/HttpIntegrationConfigurationInitializer.java
# spring-integration-webflux/src/main/java/org/springframework/integration/webflux/config/WebFluxIntegrationConfigurationInitializer.java
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