add support for github token exchange api to authenticate with the cli

[tim-thacker-nullify]

Description

Add a --github-token argument to give the CLI a github actions token which it can use to exchange for a nullify token.

Checklist

• [ ] I have added one of the patch, minor, major or no-release labels
• [ ] I have linked an issue from this repository using the Development option
• [ ] I have performed a self-review of my own code
• [ ] I have checked for redundant or commented out code
• [ ] I have commented my code where I can't make it self-documenting
• [ ] I have made corresponding changes to the documentation
• [ ] I have added any appropriate tests