Windows Credential Manager support

[epizut]

On Windows, pip is able to save credentials in the Windows Credential Manager. Unfortunately, pipenv is unable to use those credentials when addressing a private repo.

People are already familiar with pip auth (pip.ini or credential manager), so having to learn new patterns (.env file, etc) increases the pipenv learning curve.

To be more general: pipenv should be able to authenticate natively everywhere pip can. I tried playing with disable_pip_input without success, it may help with a keychain auth, but not with a local credential manager.