[FR] Allow managing of multi-factor users

**Is your feature request related to a problem? Please describe.**
It would be great if we could manage multi-factor users like described in https://cloud.google.com/identity-platform/docs/admin/manage-mfa-users

**Describe the solution you'd like**
* Add `multi_factor` property to `UserRecord`
* Add `multi_factor` arguments to `create_user` and `update_user`

**Describe alternatives you've considered**
None

**Additional context**
None