Fix IntegrationMBeanExporter logic for endpoints

Related to https://stackoverflow.com/questions/72851234/error-in-startup-application-when-using-serviceactivator-in-spring-cloud-stream

The `MessagingAnnotationPostProcessor` register an `endpoint` bean for Messaging Annotation on POJO methods.
The `IntegrationMBeanExporter` post-process such a bean and registers respective MBean.
It does this not in optimal way scanning all the `IntegrationConsumer` beans for requested `MessageHandler`
which may cause a `BeanCurrentlyInCreationException`.

* Rework the logic of the `IntegrationMBeanExporter.postProcessAbstractEndpoint()` to propagate provided endpoint
for the monitor registration to bypass application context scanning for matched name.
* Swap `equals()` for `monitor` since an `extractTarget()` may return `null`
* Some other code clean in the `IntegrationMBeanExporter`
* Change one of the `@ServiceActivator` in the configuration for the `ScatterGatherHandlerIntegrationTests`
to POJO method.
However, this didn't fail for me with original code unlike in the sample application provided in the mentioned SO thread.

**Cherry-pick to `main`**

spring-integration-jmx/src/test/java/org/springframework/integration/monitor/ScatterGatherHandlerIntegrationTests.java

@@ -333,21 +333,9 @@ public class ScatterGatherHandlerIntegrationTests {
 			return new DirectChannel();
 		}
 
-		@Bean
-		@ServiceActivator(inputChannel = "serviceChannel2")
-		public MessageHandler service2() {
-			return new AbstractReplyProducingMessageHandler() {
-
-				{
-					setOutputChannel(gatherChannel());
-				}
-
-				@Override
-				protected Object handleRequestMessage(Message<?> requestMessage) {
-					return Math.random();
-				}
-
-			};
+		@ServiceActivator(inputChannel = "serviceChannel2", outputChannel = "gatherChannel")
+		public double service2() {
+			return Math.random();
 		}
 
 	}