Improve the Saml2AuthenticationRequest object

- introduce the AssertionConsumerServiceURL attribute
- add javadoc
- align property name with SAML XML for AuthNRequest

samples/boot/saml2login/src/integration-test/java/org/springframework/security/samples/Saml2LoginIntegrationTests.java

@@ -160,6 +160,13 @@ public class Saml2LoginIntegrationTests {
 				"https://simplesaml-for-spring-saml.cfapps.io/saml2/idp/SSOService.php",
 				destination
 		);
+		String acsURL = authnRequest.getAssertionConsumerServiceURL();
+		assertEquals(
+				"AssertionConsumerServiceURL must match",
+				"http://localhost:8080/login/saml2/sso/simplesamlphp",
+				acsURL
+		);
+
 	}