R1:cac61980f904

Add "temporary tokens" to auth, for SMS codes, TOTP codes, reset codes, etc

Summary:
Ref T4398. We have several auth-related systems which require (or are improved by) the ability to hand out one-time codes which expire after a short period of time.

In particular, these are:

- SMS multi-factor: we need to be able to hand out one-time codes for this in order to prove the user has the phone.
- Password reset emails: we use a time-based rotating token right now, but we could improve this with a one…