Allow setting root attributes in Pre User Registration Actions

Feature: Allow setting or modifying root attributes in Pre User Registration actions

Description: The early release feature of Signup and Login Prompt Customizations allows for the addition of custom form fields to collect additional information on user signup. Our hope was to use this to allow for first and last name to be set during user registration. However, the Pre User Registration can only modify user_data and app_data on a user object.

Use-case: Our application relies upon users having a first and last name set in their user profile. While we could make use of the user_data field, this would require additional functionality to pull this information from custom claims instead of the OIDC standard claims of given_name and family_name. It also means that the user management interface in Auth0 would not display the correct information for first and last name, making locating users by name more difficult as the name field is being defaulted to the email address provided at signup.