R1:c731508d748a

Require MFA implementations to return a formal result object when validating factors

Summary:
Ref T13222. See PHI873. Currently, MFA implementations return this weird sort of ad-hoc dictionary from validation, which is later used to render form/control stuff.

I want to make this more formal to handle token reuse / session binding cases, and let MFA factors share more code around challenges. Formalize this into a proper object instead of an ad-hoc bundle of properties.

Test Plan:
- Answered a TOTP MFA…