Adding email as option to generate new username.

Closes #3525613

src/OpenIDConnect.php

@@ -560,7 +560,7 @@ class OpenIDConnect {
    *   A unique username.
    */
   public function generateUsername(string $sub, array $userinfo, string $client_name): string {
-    $name = 'oidc_' . $client_name . '_' . md5($sub);
+    $name = $userinfo['email'];
     $candidates = ['preferred_username', 'name'];
     foreach ($candidates as $candidate) {
       if (!empty($userinfo[$candidate])) {