fix: refactor TOTP MFA into separate methods (#1698)

## What kind of change does this PR introduce?

Refactors TOTP, Challenge, Enroll, and Verify into separate branches for
consistency with other methods and also readability.

Adds an additional check to ensure a user must own a factor in order to
challenge it.

---------

Co-authored-by: Kang Ming <kang.ming1996@gmail.com>