Allow Validator config in XML websocket namespace

This commit adds a new "validator" XML attribute to the
`<websocket:message-broker/>` element. This allows configuring a
specific Validator to be used for payload validation.

Issue: SPR-13996

spring-webmvc/src/test/java/org/springframework/web/servlet/config/MvcNamespaceTests.java

@@ -853,6 +853,7 @@ public class MvcNamespaceTests {
 		ContentNegotiationManager manager = (ContentNegotiationManager) accessor.getPropertyValue(beanName);
 		assertNotNull(manager);
 		assertSame(manager, this.appContext.getBean(ContentNegotiationManager.class));
+		assertSame(manager, this.appContext.getBean("mvcContentNegotiationManager"));
 	}
 
 	@Test