Extending the model file manager to get an arbitrary list of files

[nlathia]

The modelstore library supports uploading files that have been saved to disk -- this enables modelstore to work with frameworks that have not already been added to the library, or frameworks that are not public.

Currently, the implementation allows users to upload a single file. If the model has more than one file (as is the case with transformer models, for example), the user needs to combine them together first.

The idea here is to change the model_file.py manager to allow users to specify a directory or a list of files, and enable all of these to be added into the registry archive.