Config that controls user information updates

[adorton-adobe]

Summary

• Since introducing username updates, we need to implement a way to control which fields are updated when updating user info
• It may not always be desirable to update usernames
• By default we should allow updates of first name, last name and email address since those have historically been updatable
• This config allows username updates to be opt-in
• UST will log a warning if a user has an attribute that can potentially be updated and the config is set up not to update it

Testing Steps

• Add the following config under adobe_users

  update_attributes:
  - lastname
  - firstname
  - email
  # - username

• Run sync with update_user_info enabled
• Test the updating of different attributes with regard to the update_attriubtes list

Fixes #775