verify_ticket: allow general non-challenge tfa to be run as two step call

verify_ticket: allow general non-challenge tfa to be run as two step call

This allows for doing OTP TFA in two steps, first login with normal
credentials and get the half-logged-in ticket, then send the OTP
verification for full login, same as with u2f was already possible.

This allows for a nicer UI, as OTP fields can be shown on demand, and
do not need to be visible by default.

The old way of sending the OTP code immediately with the initial
credentials request still works for backward compatibility and as
some API user may prefer it.

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>