GUACAMOLE-78: Define a standard identifier for anonymous users.

guacamole-ext/src/main/java/org/apache/guacamole/net/auth/AuthenticatedUser.java

@@ -28,6 +28,12 @@ package org.apache.guacamole.net.auth;
  */
 public interface AuthenticatedUser extends Identifiable {
 
+    /**
+     * The identifier reserved for representing a user that has authenticated
+     * anonymously.
+     */
+    public static final String ANONYMOUS_IDENTIFIER = "";
+
     /**
      * Returns the AuthenticationProvider that authenticated this user.
      *