Add Reactive Clear-Site-Data Support

1. A new implementation of ServerHttpHeadersWriter has been created to
   add Clear-Site-Data header support.
2. A new implementation of ServerLogoutHandler has been created which
   can be configured to write response headers during logout.
3. Added unit tests for both implementations.

Fixes gh-6743

web/src/main/java/org/springframework/security/web/server/authentication/logout/HeaderWriterServerLogoutHandler.java

@@ -0,0 +1,50 @@
+/*
+ * Copyright 2002-2019 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.security.web.server.authentication.logout;
+
+import org.springframework.security.core.Authentication;
+import org.springframework.security.web.server.WebFilterExchange;
+import org.springframework.security.web.server.header.ServerHttpHeadersWriter;
+import org.springframework.util.Assert;
+
+import reactor.core.publisher.Mono;
+
+/**
+ * <p>A {@link ServerLogoutHandler} implementation which writes HTTP headers during logout.</p>
+ *
+ * @author MD Sayem Ahmed
+ * @since 5.2
+ */
+public final class HeaderWriterServerLogoutHandler implements ServerLogoutHandler {
+	private final ServerHttpHeadersWriter headersWriter;
+
+	/**
+	 * <p>Constructs a new instance using the {@link ServerHttpHeadersWriter} implementation.</p>
+
+	 * @param headersWriter a {@link ServerHttpHeadersWriter} implementation
+	 * @throws IllegalArgumentException if the argument is null
+	 */
+	public HeaderWriterServerLogoutHandler(ServerHttpHeadersWriter headersWriter) {
+		Assert.notNull(headersWriter, "headersWriter cannot be null");
+		this.headersWriter = headersWriter;
+	}
+
+	@Override
+	public Mono<Void> logout(WebFilterExchange exchange, Authentication authentication) {
+		return this.headersWriter
+				.writeHttpHeaders(exchange.getExchange());
+	}
+}

web/src/main/java/org/springframework/security/web/server/header/ClearSiteDataServerHttpHeadersWriter.java

@@ -0,0 +1,95 @@
+/*
+ * Copyright 2002-2019 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.security.web.server.header;
+
+import org.springframework.util.Assert;
+import org.springframework.web.server.ServerWebExchange;
+
+import reactor.core.publisher.Mono;
+
+import java.util.stream.Collectors;
+import java.util.stream.Stream;
+
+/**
+ * <p>Writes the {@code Clear-Site-Data} response header when the request is secure.</p>
+ *
+ * <p>For further details pleaes consult <a href="https://www.w3.org/TR/clear-site-data/">W3C Documentation</a>.</p>
+ *
+ * @author MD Sayem Ahmed
+ * @since 5.2
+ */
+public final class ClearSiteDataServerHttpHeadersWriter implements ServerHttpHeadersWriter {
+	public static final String CLEAR_SITE_DATA_HEADER = "Clear-Site-Data";
+
+	private final StaticServerHttpHeadersWriter headerWriterDelegate;
+
+	/**
+	 * <p>Constructs a new instance using the given directives.</p>
+	 *
+	 * @param directives directives that will be written as the header value
+	 * @throws IllegalArgumentException if the argument is null or empty
+	 */
+	public ClearSiteDataServerHttpHeadersWriter(Directive... directives) {
+		Assert.notEmpty(directives, "directives cannot be empty or null.");
+		this.headerWriterDelegate = StaticServerHttpHeadersWriter.builder()
+				.header(CLEAR_SITE_DATA_HEADER, transformToHeaderValue(directives))
+				.build();
+	}
+
+	@Override
+	public Mono<Void> writeHttpHeaders(ServerWebExchange exchange) {
+		if (isSecure(exchange)) {
+			return this.headerWriterDelegate
+					.writeHttpHeaders(exchange);
+		} else {
+			return Mono.empty();
+		}
+	}
+
+	/**
+	 * <p>Represents the directive values expected by the {@link ClearSiteDataServerHttpHeadersWriter}</p>.
+	 */
+	public enum Directive {
+		CACHE("cache"),
+		COOKIES("cookies"),
+		STORAGE("storage"),
+		EXECUTION_CONTEXTS("executionContexts"),
+		ALL("*");
+
+		private final String headerValue;
+
+		Directive(String headerValue) {
+			this.headerValue = "\"" + headerValue + "\"";
+		}
+
+		public String getHeaderValue() {
+			return this.headerValue;
+		}
+	}
+
+	private String transformToHeaderValue(Directive... directives) {
+		return Stream.of(directives)
+				.map(Directive::getHeaderValue)
+				.collect(Collectors.joining(", "));
+	}
+
+	private boolean isSecure(ServerWebExchange exchange) {
+		String scheme = exchange.getRequest()
+				.getURI()
+				.getScheme();
+		return scheme != null && scheme.equalsIgnoreCase("https");
+	}
+}