Allow passing the resource's version number under "resource" property

https://github.com/Gizra/restful/pull/113/files#r15042743
The git diff looks weird for some reason -- but in 7.x-1.x we have

```php
    $public_fields = $this->getPublicFields();

    if (empty($public_fields[$public_field_name]['resource'])) {
      // Field is not defined as "resource", which means it only accepts an
      // integer as a valid value.
      return $value;
    }
```

which seems to be gone now.