Determine default owner from App authentication

[dortort]

Determine owner from App authentication (app_auth) if none specified.

Suggested fix by @shrink to https://github.com/integrations/terraform-provider-github/issues/929.

[jcudit]

Seems like a good PR to ship with the upcoming v5 changes. Will circle back once the auth fixes are more solid and try to tie this effort in.

[nickfloyd]

👋 Hey Friends, this issue has been automatically marked as stale because it has no recent activity. It will be closed if no further activity occurs. Please add the Status: Pinned label if you feel that this issue needs to remain open/active. Thank you for your contributions and help in keeping things tidy!

[tekumara]

I would use this!