GUACAMOLE-289: Add getResource() functions to UserContext and AuthenticationProvider, allowing extensions to expose arbitrary REST resources/services.

extensions/guacamole-auth-duo/src/main/java/org/apache/guacamole/auth/duo/DuoAuthenticationProvider.java

@@ -62,6 +62,11 @@ public class DuoAuthenticationProvider implements AuthenticationProvider {
         return "duo";
     }
 
+    @Override
+    public Object getResource() {
+        return null;
+    }
+
     @Override
     public AuthenticatedUser authenticateUser(Credentials credentials)
             throws GuacamoleException {