Path Traversal in FRI Server /download Endpoint

@pradeeban 
File: fri/server/main.py

The download_file parameter is not sanitized:

```
download_file = request.args.get('fetch')
return send_from_directory(directory_name, download_file, as_attachment=True)
```

A request like /download/test?fetchDir=src&fetch=../../../etc/passwd can read any file on the server.