Add session attributes to SimpMessageHeaderAccessor

This change exposes the WebSocketSession attributes through a message header.
The StompSubProtocolHandler adds this to incoming messages.
For now messaging handling  methods can access the map via @Header, e.g.:

@Header(StompHeaderAccessor.SESSION_ATTRIBUTES) Map<String, Object> attrs) {

Issue: SPR-11566

spring-websocket/src/test/java/org/springframework/web/socket/messaging/StompSubProtocolHandlerTests.java

@@ -170,6 +170,7 @@ public class StompSubProtocolHandlerTests {
 		StompHeaderAccessor headers = StompHeaderAccessor.wrap(actual);
 		assertEquals(StompCommand.CONNECT, headers.getCommand());
 		assertEquals("s1", headers.getSessionId());
+		assertNotNull(headers.getSessionAttributes());
 		assertEquals("joe", headers.getUser().getName());
 		assertEquals("guest", headers.getLogin());
 		assertEquals("PROTECTED", headers.getPasscode());