[POC] Data mapper fs

PoC HERE : ```test/adapter.php```
I think file attributes are part of file metadata.

So 2 solutions IMO:
- either we provide a `getMetadata(File $file)` from the adapter (and filesystem) that provides a `Metadata` object that contains this data
- either we provide this method directly from the adapter (getModifiedTime, getCreationTime,  isReadOnly, getSize etc.)