MA-95: Update Registration endpoint to support 3rd party credentials

common/djangoapps/oauth_exchange/forms.py

common/djangoapps/oauth_exchange/tests/test_forms.py

common/djangoapps/oauth_exchange/tests/test_views.py

common/djangoapps/oauth_exchange/tests/utils.py

common/djangoapps/student/tests/test_login.py

common/djangoapps/student/views.py

common/djangoapps/third_party_auth/pipeline.py

common/djangoapps/third_party_auth/settings.py

...angoapps/third_party_auth/tests/test_change_enrollment.py

common/djangoapps/third_party_auth/tests/utils.py

lms/urls.py

openedx/core/djangoapps/user_api/tests/test_views.py