feat: Expose GitHub username and email via External Authentication

[3UR]

When using GitHub External Auth we cannot get the GitHub username and email.

This would be useful because some users have different emails and usernames on Coder compared to GitHub.

If the username and email were exposed, they could be used like this:

  git_author_name           = data.coder_external_auth.github.username
  git_author_email           = data.coder_external_auth.github.email

[stirby]

This is a great proposal, we plan to work on a design for this soon!