Option to add SteamGuard using a phone number

[DeverBa]

Description

Would be nice to add a separate option to add SteamGuard using a phone number.

[dyc3]

So as it stands right now, I can't figure out a good way to automatically determine whether or not a steam account already has a phone number set up. The previous method was pinging https://steamcommunity.com/steamguard/phoneajax with op=has_phone, but that doesn't seem to work anymore. Additionally, IPhoneService and ITwoFactorService also doesn't provide any methods to check phone number status.

[dyc3]

I've recently reverse engineered the endpoint that lets us reliably check if a phone number is on the account in #449, so this is now possible.