Move requests session to ctmods

[DavidoTek]

Currently, all ctmods share a single requests session which may contain the authorization header for GitHub. With #302, fetching compatibility tools from GitLab is now possible too. This requires ctmods to be able to choose between the GitHub and GitLab authorization key.

This PR does following changes:

• Each ctmod has its own requests session
• The ctmods will read the authorization keys from main_window.web_access_tokens and build a header using util.py#build_headers_with_authorization which is added to the local requests session

See https://github.com/DavidoTek/ProtonUp-Qt/pull/302#issuecomment-1808909601 for details

[sonic2kk]

Glad you also caught removing the header update in the DXVK ctmod's __download, I meant to bring that up :smile:

This should also make it more convenient for other ctmods to adopt the download methods we use here. My main concern with this approach in #302 was to do with maybe a higher memory cost or something, but after discussion in that PR, it seems that is a non-issue and there is no concern with each ctmod having its own requests.Session.

Looks great :-)