EZP-22960: Made DFS FS backend configurable

> Implements EZP-22960
> Status: ready

A new INI setting, eZDFSClusteringSettings.DFSBackend, is introduced that sets the backend class used by DFS. The backend instance is provided by static methods in `eZDFSFileHandlerBackendFactory`.

If a backend implements `eZDFSFileHandlerDFSFactoryBackendInterface`, it will be constructed using a `build()` static method. If it doesn't, an instance will be created using `new`.