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-context/src/main/java/org/springframework/context/ApplicationContextInitializer.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2018 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.
@@ -28,7 +28,7 @@ package org.springframework.context;
  *
  * <p>{@code ApplicationContextInitializer} processors are encouraged to detect
  * whether Spring's {@link org.springframework.core.Ordered Ordered} interface has been
- * implemented or if the @{@link org.springframework.core.annotation.Order Order}
+ * implemented or if the {@link org.springframework.core.annotation.Order @Order}
  * annotation is present and to sort instances accordingly if so prior to invocation.
  *
  * @author Chris Beams
@@ -39,6 +39,7 @@ package org.springframework.context;
  * @see org.springframework.web.servlet.FrameworkServlet#setContextInitializerClasses
  * @see org.springframework.web.servlet.FrameworkServlet#applyInitializers
  */
+@FunctionalInterface
 public interface ApplicationContextInitializer<C extends ConfigurableApplicationContext> {
 
 	/**

spring-context/src/main/java/org/springframework/validation/MessageCodeFormatter.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2018 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.
@@ -26,6 +26,7 @@ import org.springframework.lang.Nullable;
  * @see DefaultMessageCodesResolver
  * @see DefaultMessageCodesResolver.Format
  */
+@FunctionalInterface
 public interface MessageCodeFormatter {
 
 	/**