Support suspending handler methods in Spring MVC

This commit adds support for Kotlin Coroutines suspending functions to
Spring MVC, by converting those to a Mono that can then be handled by
the asynchronous request processing feature.

It also optimizes Coroutines detection with the introduction of an
optimized KotlinDetector.isSuspendingFunction() method that does not
require kotlin-reflect.

Closes gh-23611

spring-core/src/main/java/org/springframework/core/KotlinDetector.java

@@ -17,6 +17,7 @@
 package org.springframework.core;
 
 import java.lang.annotation.Annotation;
+import java.lang.reflect.Method;
 
 import org.springframework.lang.Nullable;
 import org.springframework.util.ClassUtils;
@@ -74,4 +75,20 @@ public abstract class KotlinDetector {
 		return (kotlinMetadata != null && clazz.getDeclaredAnnotation(kotlinMetadata) != null);
 	}
 
+	/**
+	 * Return {@code true} if the method is a suspending function.
+	 *
+	 * @author Sebastien Deleuze
+	 * @since 5.3
+	 */
+	public static boolean isSuspendingFunction(Method method) {
+		if (KotlinDetector.isKotlinType(method.getDeclaringClass())) {
+			Class<?>[] types = method.getParameterTypes();
+			if ((types.length > 0) && "kotlin.coroutines.Continuation".equals(types[types.length - 1].getName())) {
+				return true;
+			}
+		}
+		return false;
+	}
+
 }