Add SSE support to WebMvc.fn

This commit adds support for sending Server-Sent Events in WebMvc.fn,
through the ServerResponse.sse method that takes a SseBuilder DSL.
It also includes reference documentation.

Closes gh-25920

spring-web/src/main/java/org/springframework/http/server/DelegatingServerHttpResponse.java

@@ -0,0 +1,79 @@
+/*
+ * 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.
+ * 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.http.server;
+
+import java.io.IOException;
+import java.io.OutputStream;
+
+import org.springframework.http.HttpHeaders;
+import org.springframework.http.HttpStatus;
+import org.springframework.util.Assert;
+
+/**
+ * Implementation of {@code ServerHttpResponse} that delegates all calls to a
+ * given target {@code ServerHttpResponse}.
+ *
+ * @author Arjen Poutsma
+ * @since 5.3.2
+ */
+public class DelegatingServerHttpResponse implements ServerHttpResponse {
+
+	private final ServerHttpResponse delegate;
+
+	/**
+	 * Create a new {@code DelegatingServerHttpResponse}.
+	 * @param delegate the response to delegate to
+	 */
+	public DelegatingServerHttpResponse(ServerHttpResponse delegate) {
+		Assert.notNull(delegate, "Delegate must not be null");
+		this.delegate = delegate;
+	}
+
+	/**
+	 * Returns the target response that this response delegates to.
+	 * @return the delegate
+	 */
+	public ServerHttpResponse getDelegate() {
+		return this.delegate;
+	}
+
+	@Override
+	public void setStatusCode(HttpStatus status) {
+		this.delegate.setStatusCode(status);
+	}
+
+	@Override
+	public void flush() throws IOException {
+		this.delegate.flush();
+	}
+
+	@Override
+	public void close() {
+		this.delegate.close();
+	}
+
+	@Override
+	public OutputStream getBody() throws IOException {
+		return this.delegate.getBody();
+	}
+
+	@Override
+	public HttpHeaders getHeaders() {
+		return this.delegate.getHeaders();
+	}
+
+}