idtoken: simplify usage of `authorized_user` user credentials

**Is your feature request related to a problem? Please describe.**
I would like to support authorized_user and other methods of obtaining an idtoken. However, with the standard flow this fails with `authorized_user, use idtoken::user_account::Builder directly.`. But, If I want to use `user_account::Builder `, I need to get the `authorized_user` json value. The library uses `load_adc` for this, but htis is a private method.

This means in order to do this, I need to re-implement `load_adc`.

**Describe the solution you'd like**
Allow a method to do this automatically for me, or at least expose a mechanism to read the `authorized_user` json value from ADC

**Describe alternatives you've considered**
Copy+paste the private code

**Additional context**
Add any other context or screenshots about the feature request here.