Support querying users' GitHub IDs

[patzielinski]

See gittuf/gittuf#997. It's not super trivial for users to query a user's GitHub ID and add it to gittuf metadata. We should add support to the gittuf GitHub app to support querying user IDs on demand instead of requiring that they be added ahead of time.

[adityasaky]

Can you elaborate on the proposed workflow with the app? How would a policy writer use this feature to get someone's GitHub user ID?

[patzielinski]

I figured that a policy writer can just record the GitHub username and custom instance (if applicable) in gittuf policy, and then the app queries the appropriate GitHub endpoint to determine the username?

I originally opened a PR in the gittuf tool for a policy writer to query the ID but I recall discussing that this would be better in the app?