Authentication using phone number only

**Is your feature request related to a problem? Please describe.**
Yes - when I tried using Amplify for authenticating users using a phone number and TOTP, I ran into some issues.

**Describe the solution you'd like**
I'd like there to be another "alternative sign in" option, where you can sign up/sign in using your phone number only (and TOTP for verification on both registration and login).

**Describe alternatives you've considered**
You could use the hash of the phone number as the username, the phone number as the password, and enable MFA. However, that would be extremely hack-y, and so I'd prefer not to use it.

**Additional context**
aws-amplify/aws-sdk-android#1012 (and more specifically https://github.com/aws-amplify/aws-sdk-android/issues/1012#issuecomment-505580288).

// cc @undefobj