R1:a62f334d9503

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…