GH-10018: Implement async for Java DSL gateway()

Fixes: https://github.com/spring-projects/spring-integration/issues/10018

The `GatewayEndpointSpec` configuration of the `gateway()` operator support already an `async(true)` option.
However, it is silently ignored internally since no real async contract provided for the gateway proxy.

* Introduce the `AsyncRequestReplyExchanger` interface to use instead of `RequestReplyExchanger`,
when  `gateway()` operator is opted-in for the `async(true)`
* Use this new `AsyncRequestReplyExchanger` in the `GatewayMessageHandler` when `async(true)`
* Also, expose a `GatewayEndpointSpec.asyncExecutor(Executor)` option to support async behavior similar to the `@MessagingGateway`

src/reference/antora/modules/ROOT/pages/dsl/java-gateway.adoc

@@ -34,3 +34,6 @@ private static IntegrationFlow subFlow() {
 IMPORTANT: If the downstream flow does not always return a reply, you should set the `requestTimeout` to 0 to prevent hanging the calling thread indefinitely.
 In that case, the flow will end at that point and the thread released for further work.
 
+Starting with version 6.5, this `gateway()` operator fully supports an `async(true)` behaviour.
+Internally, an `AsyncRequestReplyExchanger` service interface is provided for the `GatewayProxyFactoryBean`.
+And since `AsyncRequestReplyExchanger` contract is a `CompletableFuture<Message<?>>`, the whole request-reply is executed in asynchronous manner.

src/reference/antora/modules/ROOT/pages/gateway.adoc

@@ -560,6 +560,12 @@ int finalResult =  result.get(1000, TimeUnit.SECONDS);
 
 For a more detailed example, see the https://github.com/spring-projects/spring-integration-samples/tree/main/intermediate/async-gateway[async-gateway] sample in the Spring Integration samples.
 
+Also, starting with version 6.5, the Java DSL `gateway()` operator fully supports an `async(true)` behaviour.
+Internally, an `AsyncRequestReplyExchanger` service interface is provided for the `GatewayProxyFactoryBean`.
+And since `AsyncRequestReplyExchanger` contract is a `CompletableFuture<Message<?>>`, the whole request-reply is executed in asynchronous manner.
+This behavior is useful, for example, in case of splitter-aggregator scenario when another flow has to be called for each item.
+However, the order is not important - only their group gathering on the aggregator after all processing.
+
 [[gateway-asynctaskexecutor]]
 === `AsyncTaskExecutor`
 

src/reference/antora/modules/ROOT/pages/whats-new.adoc

@@ -31,6 +31,9 @@ The `AbstractMessageChannel` beans now throw a special `MessageDispatchingExcept
 In general, it is a design error to try to produce a message from `afterPropertiesSet()`, `@PostConstruct` or bean definition methods.
 The `SmartLifecycle.start()` is preferred way for this kind of logic, or better to do that via inbound channel adapters.
 
+The Java DSL `gateway()` operator now fully supports an `async(true)` behavior.
+See xref:gateway.adoc[] for more information.
+
 [[x6.5-lock-request-handler-advice]]
 == The `LockRequestHandlerAdvice`