Add SameSite cookie support for servlet web servers

Update Tomcat, Jetty and Undertow `ServletWebServerFactory`
implementations so that they can write SameSite cookie attributes.

The session cookie will be customized whenever the
`server.servlet.session.cookie.same-site` property is set.

Other cookies can be customized with the new `CookieSameSiteSupplier`
interface which can be registered using `@Bean` methods.

Closes gh-20971

Co-authored-by Andy Wilkinson <wilkinsona@vmware.com>

spring-boot-project/spring-boot-docs/src/main/java/org/springframework/boot/docs/web/servlet/embeddedcontainer/customizing/samesite/MySameSiteConfiguration.java

@@ -0,0 +1,31 @@
+/*
+ * Copyright 2012-2021 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.boot.docs.web.servlet.embeddedcontainer.customizing.samesite;
+
+import org.springframework.boot.web.servlet.server.CookieSameSiteSupplier;
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+
+@Configuration(proxyBeanMethods = false)
+public class MySameSiteConfiguration {
+
+	@Bean
+	public CookieSameSiteSupplier applicationCookieSameSiteSupplier() {
+		return CookieSameSiteSupplier.ofLax().whenHasNameMatching("myapp.*");
+	}
+
+}