Auth with Github

[aslamplr]

Auth flow integration with Github OAuth. Open Login/Auth screen on the browser. Store credentials optionally and no need to pass --auth_token arg each time.

References

• https://developer.github.com/apps/building-oauth-apps/authorizing-oauth-apps/#web-application-flow
• https://github.com/cli/cli/blob/trunk/auth/oauth.go
• https://github.com/cli/cli/blob/658d548c5e690b4fb4dd6ac06d4b798238b6157f/internal/config/config_setup.go#L54
• https://github.com/cli/cli/blob/658d548c5e690b4fb4dd6ac06d4b798238b6157f/internal/config/config_setup.go#L74
• https://github.com/amodm/webbrowser-rs

[aslamplr]

Follow https://github.com/cli/cli/issues/297 for Auth in server like environments where there is no GUI and web browser.