Add missing Mailgun 'endpoint' option

Counterpart to https://github.com/laravel/framework/pull/25010

config/services.php

@@ -17,6 +17,7 @@ return [
     'mailgun' => [
         'domain' => env('MAILGUN_DOMAIN'),
         'secret' => env('MAILGUN_SECRET'),
+        'endpoint' => env('MAILGUN_ENDPOINT', 'api.mailgun.net'),
     ],
 
     'ses' => [