Support push 2FA in internal API endpoint with polling (FortiToken Cloud)

This is a follow-up to #322595 (closed). To extend the capabilities of checking push auth result, we should add support for providers (like FortiToken Cloud) that does the check async, as opposed to using blocking calls.

The response should return an URL where the client can check the push auth result via polling. By introducing a new /two_factor_push_result internal API endpoint, /two_factor_check would return the URL pointing to that endpoint where gitlab-shell can continue polling.