[POC] Data mapper fs

PoC HERE : ```test/adapter.php```
Passing a File instance allows to have a smarter approach with the actual file retrieval on the FS.

Higher API (like FileSystem) will have fetch method which accept string $key:

```
class Filesystem
{
    public function get($file)
    {
        if (!$file instanceof File) {
            $file = $this->factory->createFile($file);
        }
        
        return $this->adapter->get($file);
    }
}