[Ficus] sync custom forms fields on SSO registration

[melvinsoft]

This solution was designed for Trinity initially, but I could be useful for any customer using registration fields.

The problem:
When we are using SSO, in most of the cases, customers wants to use auto-registration, this means when a user clicks on sign in or register, is redirected to the Identity Provider (OAuth or SAML), after the user successfully logs in, is redirected back to edX with the auth request, and the user attributes. By default edX automatically populate only default registration fields. So when we are using Extra registration fields, there is no way to sync those attributes. Besides the problem of not have this fields in our database, if some of the extra fields are required, the auto-registration workflow will break.

The solution:
This PR adds a new settings: CUSTOM_SSO_FIELDS_SYNC by default in False, but is expected to be a dictionary, when we can list all the extra registration form fields that we want to map. The code changes are pretty small, basically they check if this dict isn't false, and maps the attributes using the same default logic.

[melvinsoft]

(8) Trinity SSO: Sync "District" field in OAuth auto registration.

[tkeemon]

LGTM 👍

[bryanlandia]

Hey Maxi, I know you already merged this, but this might be a good one to push upstream if some tests are added and you move the changes from envs/*_appsembler.py to envs/*.py

[melvinsoft]

@bryanlandia Good idea, I'm going to create a card on the inbox to address this.