SPNEGO authentication support

[evgenyrahman]

Describe the feature or problem you’d like to solve

Some GitHub CLI users rely on the use of an authenticated proxy which has the necessary proxy rules allowing access to github.com. This authenticated proxy may use NTLM or Kerberos, which use SPNEGO authentication.

When a user is behind a proxy such as this, the gh CLI in its current form fails even if the login via the browser was successful:

> gh auth status
You are not logged into any GitHub hosts. Run gh auth login to authenticate.

> gh auth login
? What account do you want to log into? GitHub.com
? What is your preferred protocol for Git operations? HTTPS
? Authenticate Git with your GitHub credentials? Yes
? How would you like to authenticate GitHub CLI? Login with a web browser

Press Enter to open github.com in your browser...
failed to authenticate via web browser: HTTP 403

This is because most URLs for github.com are blocked by the proxy unless the user authenticates with the proxy via SPNEGO

The curl equivalent is 

```bash
curl -x http://proxy.example.com:PORT/ --proxy-negotiate -U

Proposed solution

How will it benefit CLI and its users?

Adding this support will unblock CLI use for GitHub users who are behind a proxy that uses SPNEGO-based authentication

[evgenyrahman]

This may help with adding SPNEGO support https://github.com/jcmturner/gokrb5

[andyfeller]

Thanks for opening up this suggestion, @evgenyrahman! ❤ There's more about this need and how the intended audience needing it that'd be helpful to understand:

1. How is git authenticating with SPNEGO?
2. What other CLI applications are being used that support SPNEGO natively?
3. Have you looked at tools like spnego-proxy which use gokrb5 as a standalone solution for any CLI application?

[evgenyrahman]

Thanks @andyfeller I'll try to find out more and report back