[feat] Implement Login to GitHub with 2FA Support

[yixinNB]

I'm attempting to clone a private GitHub repository via HTTPS, as mandated by my work requirements. However, I'm encountering difficulties with the login process.

I have attempted to use Cred::userpass_plaintext as a credential callback to provide my username and password, but I've realized that this approach doesn't work when two-factor authentication (2FA) is enabled on the GitHub account. Given this scenario, I need to find an alternative method to authenticate and clone the private repository, while still adhering to the HTTPS protocol. Are there any recommended practices or solutions that can help me achieve this, considering the 2FA requirement?

[qaqland]

From my personal point of view, this feature will only be added if the upstream libgit2 does it.