Add datagrid pager serialization config

I've added datagrid pager serialization config because I plan to return a pager instead of a raw collection for all paginable API resources.

I also had to configure jms serializer to tell it to load this config:

```
jms_serializer:
    metadata:
        directories:
            - { path: %kernel.root_dir%/../vendor/sonata-project/admin-bundle/Resources/config/serializer, namespace_prefix: 'Sonata\AdminBundle\Datagrid' }
```