added feature to use a symfony service as a volumeDriver

This will only work after https://github.com/helios-ag/ElFinderPHP/pull/2 is merged.

With this change you can use a Symfony service as a volumeDriver. 
The service should however be an instance of the FM\ElFinderPHP\Driver\ElFinderVolumeDriver class. This check is to ensure the service is a valid ElFinder VolumeDriver.

To configure a root with a service-driver you can simply use the service id as the drive key:

```yaml
fm_elfinder:
    connector:
        roots:
            uploads:
                driver: elfinder.driver.filesystem
                path: uploads
```

This means that if you add the service definition:

```xml
<service id="elfinder.driver.filesystem" class="FM\ElFinderPHP\Driver\ElFinderVolumeLocalFileSystem" />
```

It should work just fine. This feature does not break or change any existing logic, so using LocalFileSystem as the driver should work just as well.