Declare interfaces as @FunctionalInterface where feasible

This commit declares each of the following public interfaces as a
@FunctionalInterface.

- org.springframework.context.ApplicationContextInitializer
- org.springframework.test.web.servlet.DispatcherServletCustomizer
- org.springframework.validation.MessageCodeFormatter
- org.springframework.util.IdGenerator
- org.springframework.beans.factory.config.YamlProcessor.MatchCallback
- org.springframework.beans.factory.config.YamlProcessor.DocumentMatcher

Closes gh-25580

spring-core/src/main/java/org/springframework/util/IdGenerator.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2015 the original author or authors.
+ * Copyright 2002-2020 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.
@@ -19,11 +19,12 @@ package org.springframework.util;
 import java.util.UUID;
 
 /**
- * Contract for generating universally unique identifiers {@link UUID (UUIDs)}.
+ * Contract for generating universally unique identifiers ({@link UUID UUIDs}).
  *
  * @author Rossen Stoyanchev
  * @since 4.0
  */
+@FunctionalInterface
 public interface IdGenerator {
 
 	/**