Consider adding "display_name" for each challenge_type

[PieterKas]

Scenario:

Users may have multiple out-of-band proofs on a single account, in two dimensions. Users can have an email, a phone number, and an authenticator app. And, users can also have multiple of any of these factors – multiple email addresses, multiple phone numbers, and authenticator apps on multiple devices. In the case of say, multiple phones for SMS codes, we need to be able to let them pick which one to use. One option is to add a "display_name" to the "challenge type" to help the end user select which one to pick.