feat: add associate_by_email in auth pipeline

We want to associate learners by email provided by the IdP. This would
work in the usecase when a learner already has an account and is trying
to login via SAML SSO. Right now if a learner with an existing account
would try to login via a new SAML SSO which is not associated to their
account yet, they will be prompted to enter their credentials, since the
associate_by_email_if_saml pipeline step only works if enterprise app
is enabled and a learner is an enterprise user. The client is migrating
to the new SSO, and we want the learners to not have to enter their
credentials.

Author: Cup0fCoffee

common/djangoapps/third_party_auth/settings.py

@@ -53,6 +53,7 @@ def apply_settings(django_settings):
         'social_core.pipeline.social_auth.social_uid',
         'social_core.pipeline.social_auth.auth_allowed',
         'social_core.pipeline.social_auth.social_user',
+        'social_core.pipeline.social_auth.associate_by_email',
         'common.djangoapps.third_party_auth.pipeline.associate_by_email_if_login_api',
         'common.djangoapps.third_party_auth.pipeline.associate_by_email_if_saml',
         'common.djangoapps.third_party_auth.pipeline.associate_by_email_if_oauth',