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/spring-context.gradle

@@ -30,6 +30,7 @@ dependencies {
 	optional("org.hibernate:hibernate-validator")
 	optional("org.jetbrains.kotlin:kotlin-reflect")
 	optional("org.jetbrains.kotlin:kotlin-stdlib")
+	optional("org.jetbrains.kotlinx:kotlinx-coroutines-core")
 	optional("org.reactivestreams:reactive-streams")
 	testFixturesApi("org.junit.jupiter:junit-jupiter-api")
 	testFixturesImplementation(testFixtures(project(":spring-beans")))