fix(third-party-auth): Need to account for additional required demographic fields for BigCommerce provider.

[ztraboo]

This relates to social-auth change here.
CUCWD/social-auth-backend-bigcommerce#5

When creating an account with third-party auth provider BigCommerce on EducateWorkforce it appears that new required fields set by platform setting REGISTRATION_EXTRA_FIELDS were causing the automatic account creation using Skip registration form set to True to fail.

This fix makes sure that all required fields get a default value of prefer-not-to-say or we override the required field to optional using third-party provider Other settings below to ignore the required field and provide a null value. In this case the year_of_birth field didn't have a prefer-not-to-say value by default since it was an Integer field.

Third-Party Auth Provider > Option settings

{
    "REGISTRATION_EXTRA_FIELDS": {
        "year_of_birth": "optional"
    },
    ...
}

We could have optionally updated the BigCommerce registration account creation fields to account for these required fields that we are defaulting to prefer-not-to-say value, however, we'd have to get with the BigCommerce storefront administrator to make this change occur. For now we're going to keep this hidden as a requirement.

BigCommerce Article – Adding and Editing Fields in the Account Signup Form
https://support.bigcommerce.com/s/article/Editing-Form-Fields?language=en_US