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 prompt wrong (nothing, bad value).
• Answered a TOTP MFA prompt properly.
• Added new TOTP MFA, survived enrollment.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13222

Differential Revision: https://secure.phabricator.com/D19885