GH-3204: Add DSL intercept() operator

Fixes https://github.com/spring-projects/spring-integration/issues/3204

* Add an `intercept(ChannelInterceptor...)` method into `BaseIntegrationFlowDefinition` 
to register one or more channel interceptors at the current flow position.
* refactor to reuse `InterceptableChannel` creation from `wireTap`
* document the new operator

spring-integration-core/src/main/java/org/springframework/integration/dsl/BaseIntegrationFlowDefinition.java

@@ -90,6 +90,7 @@ import org.springframework.lang.Nullable;
 import org.springframework.messaging.Message;
 import org.springframework.messaging.MessageChannel;
 import org.springframework.messaging.MessageHandler;
+import org.springframework.messaging.support.ChannelInterceptor;
 import org.springframework.messaging.support.InterceptableChannel;
 import org.springframework.util.Assert;
 import org.springframework.util.CollectionUtils;
@@ -109,6 +110,7 @@ import reactor.util.function.Tuple2;
  * @author Artem Bilan
  * @author Gary Russell
  * @author Gabriele Del Prete
+ * @author Tim Feuerbach
  *
  * @since 5.2.1
  *
@@ -179,6 +181,24 @@ public abstract class BaseIntegrationFlowDefinition<B extends BaseIntegrationFlo
 		return this.currentMessageChannel;
 	}
 
+	/**
+	 * Return the current channel if it is an {@link InterceptableChannel}, otherwise register a new implicit
+	 * {@link DirectChannel} in the flow and return that one.
+	 * @return the current channel after the operation
+	 */
+	protected InterceptableChannel currentInterceptableChannel() {
+		MessageChannel currentChannel = getCurrentMessageChannel();
+		if (currentChannel instanceof InterceptableChannel) {
+			return (InterceptableChannel) currentChannel;
+		}
+		else {
+			DirectChannel newCurrentChannel = new DirectChannel();
+			channel(newCurrentChannel);
+			setImplicitChannel(true);
+			return newCurrentChannel;
+		}
+	}
+
 	protected void setImplicitChannel(boolean implicitChannel) {
 		this.implicitChannel = implicitChannel;
 	}
@@ -488,14 +508,9 @@ public abstract class BaseIntegrationFlowDefinition<B extends BaseIntegrationFlo
 	 */
 	public B wireTap(WireTapSpec wireTapSpec) {
 		WireTap interceptor = wireTapSpec.get();
-		MessageChannel currentChannel = getCurrentMessageChannel();
-		if (!(currentChannel instanceof InterceptableChannel)) {
-			currentChannel = new DirectChannel();
-			channel(currentChannel);
-			setImplicitChannel(true);
-		}
+		InterceptableChannel currentChannel = currentInterceptableChannel();
 		addComponent(wireTapSpec);
-		((InterceptableChannel) currentChannel).addInterceptor(interceptor);
+		currentChannel.addInterceptor(interceptor);
 		return _this();
 	}
 
@@ -2829,6 +2844,26 @@ public abstract class BaseIntegrationFlowDefinition<B extends BaseIntegrationFlo
 		return handle(new ServiceActivatingHandler(triggerAction, "trigger"), endpointConfigurer);
 	}
 
+	/**
+	 * Add one or more {@link ChannelInterceptor} implementations
+	 * to the current {@link #currentMessageChannel}, in the given order, after any interceptors already registered.
+	 * @param interceptorArray one or more {@link ChannelInterceptor}s.
+	 * @return the current {@link BaseIntegrationFlowDefinition}.
+	 * @throws IllegalArgumentException if one or more null arguments are provided
+	 * @since 5.3
+	 */
+	public B intercept(ChannelInterceptor... interceptorArray) {
+		Assert.notNull(interceptorArray, "'interceptorArray' must not be null");
+		Assert.noNullElements(interceptorArray, "'interceptorArray' must not contain null elements");
+
+		InterceptableChannel currentChannel = currentInterceptableChannel();
+		for (ChannelInterceptor interceptor : interceptorArray) {
+			currentChannel.addInterceptor(interceptor);
+		}
+
+		return _this();
+	}
+
 	/**
 	 * Populate a {@link FluxMessageChannel} to start a reactive processing for upstream data,
 	 * wrap it to a {@link Flux}, apply provided {@link Function} via {@link Flux#transform(Function)}