conf.yml

Acme\Uuid:
    type: embeddable
    id:
        uuid:
            type: string
            length: 36

uuid.php

<?php

namespace Acme

class Uuid
{
    private $uuid;

    public function __construct($uuid)
    {
        $this->uuid = $uuid;
    }

    public function __toString()
    {
        return (string) $this->uuid;
    }
}