Initial implementation

> Implements [EZP-22967](https://jira.ez.no/browse/EZP-22967)

Initial working implementation of the Dispatcher FSBackend.

The Dispatcher uses two configuration directives:
- a `DefaultBackend`, by default set to `eZDFSFileHandlerDFSBackend`, the original one
- a map of `<filepath> => handler`
Yes, fixing.