FR: allow front-end authentication proxy

**Environment**: Testing

- CTFd Version/Commit: 3.7.1
- Operating System: Linux
- Web Browser and Version: Firefox

I am trying to get CTFd running behind [OAuth2-proxy](https://oauth2-proxy.github.io/oauth2-proxy/) so I can get SSO from my company.
OAuth2-proxy will send an HTTP header of 'HTTP_X_FORWARDED_EMAIL' with the email address of the user that successfully authenticated. I would like to be able to have CTFd accept that header, and automatically create/login the user so they don't need to create an account after they are already SSO authenticated.