Add Coroutines support for @Cacheable

This commit adds Coroutines support for `@Cacheable`.

It also refines SimpleKeyGenerator to ignore Continuation
parameters (Kotlin does not allow to have the same method
signature with both suspending and non-suspending variants)
and refines
org.springframework.aop.framework.CoroutinesUtils.awaitSingleOrNull
in order to wrap plain value to Mono.

Closes gh-31412

spring-context/src/test/java/org/springframework/cache/interceptor/SimpleKeyGeneratorTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2020 the original author or authors.
+ * Copyright 2002-2023 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -16,9 +16,12 @@
 
 package org.springframework.cache.interceptor;
 
+import java.lang.reflect.Method;
+
 import org.junit.jupiter.api.Test;
 
 import org.springframework.core.testfixture.io.SerializationTestUtils;
+import org.springframework.util.ReflectionUtils;
 
 import static org.assertj.core.api.Assertions.assertThat;
 
@@ -28,6 +31,7 @@ import static org.assertj.core.api.Assertions.assertThat;
  * @author Phillip Webb
  * @author Stephane Nicoll
  * @author Juergen Hoeller
+ * @author Sebastien Deleuze
  */
 public class SimpleKeyGeneratorTests {
 
@@ -126,7 +130,8 @@ public class SimpleKeyGeneratorTests {
 
 
 	private Object generateKey(Object[] arguments) {
-		return this.generator.generate(null, null, arguments);
+		Method method = ReflectionUtils.findMethod(this.getClass(), "generateKey", Object[].class);
+		return this.generator.generate(this, method, arguments);
 	}
 
 }