Add a skeleton for configurable MFA provider types

Summary:
Ref T13222. Ref T13231. See PHI912. I'm planning to turn MFA providers into concrete objects, so you can disable and configure them.

Currently, we only support TOTP, which doesn't require any configuration, but other provider types (like Duo or Yubikey OTP) do require some configuration (server URIs, API keys, etc). TOTP could also have some configuration, like "bits of entropy" or "allowed window size" or whatever, if we want.

Add concrete objects for this and standard transaction / policy / query support. These objects don't do anything interesting yet and don't actually interact with MFA, this is just skeleton code for now.

Test Plan:

Reviewers: amckinley

Reviewed By: amckinley

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13231, T13222

Differential Revision: https://secure.phabricator.com/D19935