Support external credential managers

[zooba]

(This proposal is a counterpart to pypa/pip#4475 for pip - enable the same authentication methods for publishing/installing from private feeds)

See https://git-scm.com/docs/api-credentials and https://git-scm.com/docs/git-credential for overviews of git's protocol for using external credential managers.

For example, we would want to offer a credential manager that generates temporary tokens for upload to a private feed, so that we can use other authorization besides the user's name and password on private feeds.

If you have any preferences for how to integrate this, happy to take suggestions. Hopefully it can be achieved in the same way for both pip and twine.

[zooba]

This already works - twine uses keyring for this purpose. (But I'm about to open a new issue for supporting the new keyring API I'm adding)