Resolves #260 and #378

This PR adds support for federated authentication with Apple Business Manager. During the login flow, after the user has entered their Apple ID, the app now sends a network request to https://idmsa.apple.com/appleauth/auth/federate/validate to check whether the provided Apple ID uses federated auth. If so, instead of prompting for the user's password, the app now provides instructions for completing auth. The app opens a web browser where the user completes authentication with their identity provider and is redirected to a blank page. Finally, the user copies the URL from the browser, pastes it into their terminal, and can perform authenticated requests.