Support Kotlin parameter default values in handler methods

This commit adds support for Kotlin parameter default values
in handler methods. It allows to write:
@RequestParam value: String = "default"
as an alternative to:
@RequestParam(defaultValue = "default") value: String

Both Spring MVC and WebFlux are supported, including on
suspending functions.

Closes gh-21139

spring-web/src/test/java/org/springframework/web/method/support/StubArgumentResolver.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2018 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.
@@ -47,7 +47,7 @@ public class StubArgumentResolver implements HandlerMethodArgumentResolver {
 		this(valueType, null);
 	}
 
-	public StubArgumentResolver(Class<?> valueType, Object value) {
+	public StubArgumentResolver(Class<?> valueType, @Nullable Object value) {
 		this.valueType = valueType;
 		this.value = value;
 	}