Download as string

Add function to file_handler that allows for downloading output as a string:

1. The function should probably unzip the output in memory
2. As multiple files can be archived into zip, the file should probably return Dict[str] (filename to output).
3. Non-text data should be handled as a vector of bytes or ignored (we rarely have e.q. images as output).
4. The function should be named something like download_as_dict (to be determined)