Callback
When a 3rd party redirects back to the platform at the redirect_uri provided, it
may pass back additional URL parameters. Those parameters can be accessed
during the auth flow via the callback resolver.
Given the following URL constructed by a 3rd party:
https://authbridge-stage.auctane.com/authflow/fancy_integration/accept?code=the-code&foo=bar
foo=bar can be accessed by the template expression {callback:foo} which
renders bar.