R1:0fcff782533a

Convert user MFA factors to point at configurable "MFA Providers", not raw "MFA Factors"

Summary:
Ref T13222. Users configure "Factor Configs", which say "I have an entry on my phone for TOTP secret key XYZ".

Currently, these point at raw implementations -- always "TOTP" in practice.

To support configuring available MFA types (like "no MFA") and adding MFA types that need some options set (like "Duo", which needs API keys), bind "Factor Configs" to a "Factor Provider" instead.

In the future, several…