feat: allow fogg to authenticate with GH OAuth token

[jakeyheath]

Summary

This PR introduces a feature to fogg that allows the user to provide a Github OAuth token when doing module source downloading instead of SSH keys. This is handy for bot users and OAuth automation that use Fogg in the backend and don't have SSH keys. This should allow someone to do the following:

FOGG_GITHUBTOKEN=<token> fogg apply

Where the FOGG_GITHUBTOKEN is a personal access token or a token received through an OAuth flow.

Test Plan

• Unit tests

References

(Optional) Additional links to provide more context.