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-test/src/main/java/org/springframework/test/web/servlet/DispatcherServletCustomizer.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2016 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.
@@ -25,6 +25,7 @@ import org.springframework.web.servlet.DispatcherServlet;
  * @author Stephane Nicoll
  * @since 4.3.4
  */
+@FunctionalInterface
 public interface DispatcherServletCustomizer {
 
 	/**