Add missing constant

Author: tvdijen

src/SAML11/Constants.php

@@ -66,6 +66,11 @@ class Constants extends \SimpleSAML\XMLSecurity\Constants
      */
     public const AC_UNSPECIFIED = 'urn:oasis:names:tc:SAML:1.0:am:unspecified';
 
+    /**
+     * Artifact subject confirmation method.
+     */
+    public const CM_ARTIFACT = 'urn:oasis:names:tc:SAML:1.0:cm:artifact';
+
     /**
      * Bearer subject confirmation method.
      */