Add validation support into WebFlux Inbound (#2977)

* Add validation support into WebFlux Inbound

See https://stackoverflow.com/questions/56729517/spring-integration-webflux-validate-input-json-body

* Introduce a `WebFluxInboundEndpoint.validator` option to validate
a request payload after conversion HTTP request into a `Publisher`
* Expose a `WebFluxInboundEndpointSpec.validator()` option
* Introduce a `IntegrationWebExchangeBindException` to extend a
`WebExchangeBindException` for Spring Integration use-case.
such an exception is used in the `ResponseStatusExceptionHandler`
extensions, like one in Spring Boot for proper response rendering
* Test and document the solution

* * Remove unused imports from IntegrationWebExchangeBindException

spring-integration-http/src/main/java/org/springframework/integration/http/support/IntegrationWebExchangeBindException.java

@@ -0,0 +1,65 @@
+/*
+ * Copyright 2019 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.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.integration.http.support;
+
+import org.springframework.validation.BindingResult;
+import org.springframework.validation.ObjectError;
+import org.springframework.web.bind.support.WebExchangeBindException;
+
+/**
+ * A {@link WebExchangeBindException} extension for validation error with a failed
+ * message context.
+ * We can't rely on the default {@link WebExchangeBindException} behavior since
+ * there is no POJO method invocation in Spring Integration Web endpoint implementations.
+ *
+ * @author Artem Bilan
+ *
+ * @since 5.2
+ */
+@SuppressWarnings("serial")
+public class IntegrationWebExchangeBindException extends WebExchangeBindException {
+
+	private final String endpointId;
+
+	private final Object failedPayload;
+
+	public IntegrationWebExchangeBindException(String endpointId, Object failedPayload,
+			BindingResult bindingResult) {
+
+		super(null, bindingResult); // NOSONAR - we ignore a MethodParameter in favor of payload and endpoint context
+		this.endpointId = endpointId;
+		this.failedPayload = failedPayload;
+	}
+
+
+	@Override
+	public String getMessage() {
+		BindingResult bindingResult = getBindingResult();
+		StringBuilder sb = new StringBuilder("Validation failed for payload ")
+				.append(this.failedPayload)
+				.append(" in the endpoint ")
+				.append(this.endpointId)
+				.append(", with ")
+				.append(bindingResult.getErrorCount())
+				.append(" error(s): ");
+		for (ObjectError error : bindingResult.getAllErrors()) {
+			sb.append("[").append(error).append("] ");
+		}
+		return sb.toString();
+	}
+
+}