Expose client in connection and manager

This would allow things like this :

```php
$factory = new \Rizeway\OREM\Config\Factory(__DIR__ . DIRECTORY_SEPARATOR . 'OREM' . DIRECTORY_SEPARATOR . 'Mappings', 'https://api.domain.tld/');
$manager = $factory->getManager();
$client = $manager->getConnection()->getClient();

$client->getDefaultHeaders()->add('Authorization', 'AUTH_TOKEN');
```