feat: add "GET ../files"

[krish8484]

Description

• Endpoint returns ToolFile objects of all files of a tool version associated with a given descriptor
• All descriptor, test and "other" files (all files except CONTAINERFILEs) are required to be associated with a descriptor type (e.g., NFL, CWL) and pointers are returned via this endpoint (to be retrieved via GET /tools/{id}/versions/{version_id}/{type}/descriptor/{relative_path})
• Support for application/zip response not yet implemented

Fixes #28

[krish8484]

All of the things in this PR has been implemented and tested except one. I was not sure of how exactly we want the application/zippart, therefore I couldn't implement it. Everything else apart from application/zip looks OK locally.

[uniqueg]

Was a really good start. Unfortunately found some issue with the schema so needed some refactoring. Also decided to go with the more standard requests package to handle the calls to retrieve content. This will need some work in the future as it's a potential security risk and also all kinds of things can go wrong (file ends up to be huuuuge, not a text file, connection times out etc...). But for now, where we'll manually get the files in and can double check that all will be fine, it should be good enough