Make the DFS FS backend configurable

> Implements [EZP-22960](https://jira.ez.no/browse/EZP-22960)
> Based on [EZP-22966](https://jira.ez.no/browse/EZP-22966) that adds the DFSBackend interface

A new INI setting in `file.ini`, `eZDFSClusteringSettings.DFSBackend`, is introduced, that sets the FS backend class used by DFS.

The backend instance is requested using by `eZDFSFileHandlerBackendFactory::build()`.

If a backend implements `eZDFSFileHandlerFactoryDFSBackendInterface`, it will be constructed using a `factory()` static method. If it doesn't, like the native `eZDFSFileHandlerDFSBackend`, the instance will be created using `new`.