adding user creation prevention flag

Author: adl-trey

auth.php

@@ -146,6 +146,11 @@ private function attempt_jwt_login() {
 
         if (!$userExists) {
 
+            $preventAutoUserCreation = $this->prevent_auto_user_creation();
+            if ($preventAutoUserCreation) {
+                return;
+            }
+
             $username = $this->get_expected_username($payload);
             $password = null;
 
@@ -361,6 +366,10 @@ private function decode_base_64($encodedStr) {
         return base64_decode($b);
     }
 
+    private function prevent_auto_user_creation() {
+        return $this->has_env_bool("MOODLE_JWT_PREVENT_AUTO_USER_CREATION");
+    }
+
     private function has_env_bool($variableName) {
         $value = getenv($variableName);
         $exists = $value != false;