[Order]WIP order identities

This PR is an attempt to allow to add identities to an order.  For instance, when an order is synched to an external system such as an ERP system, you would want to track the ERP order id.  Similary if you would import orders from an external party (for instance ebay orders) you would want to track a reference to the ebay id.  

 Q             | A
| ------------- | ---
| Bug fix?      | [no]
| New feature?  | [yes]
| BC breaks?    | [no]
| Deprecations? | [no]
| Tests pass?   | [no]
| License       | MIT

How it currently can be used : 

$identity = new Identity();
$identity->setName('ebayId');
$identity->setValue('123213213213');
$order->addIdentity($identity);

$identity = new Identity();
$identity->setName('ERP_SAP_ID);
$identity->setValue('4000123213');
$order->addIdentity($identity);

Identities are stored in table sylius_order_identities