Add serializer config for CustomerRecommendation

Fix critical exception Resources are not supported in serialized data. Path: Monolog\Handler\StreamHandler -> Symfony\Bridge\Monolog\Logger -> Symfony\Component\Cache\Adapter\PhpFilesAdapter -> Symfony\Component\Cache\Adapter\TraceableAdapter -> Symfony\Component\Cache\Adapter\PhpArrayAdapter -> Doctrine\Common\Annotations\PsrCachedReader -> Doctrine\ORM\Mapping\Driver\AnnotationDriver -> Doctrine\Persistence\Mapping\Driver\MappingDriverChain -> Sylius\Bundle\ResourceBundle\Doctrine\ResourceMappingDriverChain -> Doctrine\Bundle\DoctrineBundle\Mapping\MappingDriver -> Doctrine\ORM\Configuration -> Doctrine\ORM\EntityManager -> Tavy315\SyliusRecommendationsPlugin\Entity\RecommendationGroup -> Tavy315\SyliusRecommendationsPlugin\Entity\CustomerRecommendation