feat: allow custom gravatar endpoints (#217)

* add custom gravatar registry env

* add default val

* use gravatar.com as default val

plugins/gravatar/bootstrap.php

@@ -9,7 +9,8 @@
         }
 
         $hashed = md5(strtolower(trim($user->email)));
-
-        return "https://www.gravatar.com/avatar/$hashed";
+        $registry = env('GRAVATAR_REGISTRY', 'https://www.gravatar.com/avatar/');
+
+        return "$registry$hashed";
     });
 };