R1:b8cbfda07ce6

Track MFA "challenges" so we can bind challenges to sessions and support SMS and other push MFA

Summary:
Ref T13222. See PHI873. Ref T9770.

Currently, we support only TOTP MFA. For some MFA (SMS and "push-to-app"-style MFA) we may need to keep track of MFA details (e.g., the code we SMS'd you). There isn't much support for that yet.

We also currently allow free reuse of TOTP responses across sessions and workflows. This hypothetically enables some "spyglass" attacks where you look at someone's phone and…