Add request binding to functional endpoints

This commit introduces form binding to ServerRequest in both WebMVC.fn
and WebFlux.fn's, in the form of a bind(Class) method.

Closes gh-25943

spring-test/src/main/java/org/springframework/mock/web/reactive/function/server/MockServerRequest.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2021 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.
@@ -29,6 +29,7 @@ import java.util.Map;
 import java.util.Optional;
 import java.util.OptionalLong;
 import java.util.concurrent.ConcurrentHashMap;
+import java.util.function.Consumer;
 
 import reactor.core.publisher.Flux;
 import reactor.core.publisher.Mono;
@@ -48,6 +49,7 @@ import org.springframework.util.Assert;
 import org.springframework.util.CollectionUtils;
 import org.springframework.util.LinkedMultiValueMap;
 import org.springframework.util.MultiValueMap;
+import org.springframework.web.bind.WebDataBinder;
 import org.springframework.web.reactive.function.BodyExtractor;
 import org.springframework.web.reactive.function.server.HandlerStrategies;
 import org.springframework.web.reactive.function.server.ServerRequest;
@@ -219,6 +221,20 @@ public final class MockServerRequest implements ServerRequest {
 		return (Flux<S>) this.body;
 	}
 
+	@Override
+	@SuppressWarnings("unchecked")
+	public <T> Mono<T> bind(Class<T> bindType) {
+		Assert.state(this.body != null, "No body");
+		return (Mono<T>) this.body;
+	}
+
+	@Override
+	@SuppressWarnings("unchecked")
+	public <T> Mono<T> bind(Class<T> bindType, Consumer<WebDataBinder> dataBinderCustomizer) {
+		Assert.state(this.body != null, "No body");
+		return (Mono<T>) this.body;
+	}
+
 	@Override
 	public Map<String, Object> attributes() {
 		return this.attributes;