R1:f3340c633562

Allow different MFA factor types (SMS, TOTP, Duo, ...) to share "sync" tokens when enrolling new factors

Summary:
Depends on D20019. Ref T13222. Currently, TOTP uses a temporary token to make sure you've set up the app on your phone properly and that you're providing an answer to a secret which we generated (not an attacker-generated secret).

However, most factor types need some kind of sync token. SMS needs to send you a code; Duo needs to store a transaction ID. Turn this "TOTP" token into an "MFA…