[7.x] Allow configuring the timeout for the smtp driver (#5262)

* Allow configuring the timeout for the smtp driver

The default is the same as in `\Swift_Transport_EsmtpTransport::$params`

* Corrected default

* Update mail.php

Co-authored-by: Graham Campbell <GrahamCampbell@users.noreply.github.com>
Co-authored-by: Taylor Otwell <taylor@laravel.com>

config/mail.php

@@ -41,6 +41,7 @@ return [
             'encryption' => env('MAIL_ENCRYPTION', 'tls'),
             'username' => env('MAIL_USERNAME'),
             'password' => env('MAIL_PASSWORD'),
+            'timeout' => null,
         ],
 
         'ses' => [