Add ChildInterface and ChildExtension

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT

Previously discussed in https://github.com/sonata-project/SonataDoctrinePhpcrAdminBundle/pull/236

To make it work, it currently requires the following config :

```
sonata_admin:
    extensions:
        cmf_core.admin_extension.child:
            implements:
                - Symfony\Cmf\Bundle\CoreBundle\Model\ChildInterface
```

If it's ok, I will update the docs.