Add support for automatically extracting models that are archives

[brimoor]

Not all models can be encapsulated by a single file. For example, one may want to deploy a TF model in SavedModel format, which is defined by a directory.

In this case, the published model will be an archive, and it would be convenient if ETA automatically handled unarchiving the data (via eta.core.utils.extract_archive()) when downloading the model. ETA should also handle cleanup of such models when the corresponding archive would ordinarily be deleted.