Add support for OpenEDX instances with external identity provider / SSO

[benoit74]

As encountered in #170, OpenEDX instances might be configured with an external identity provider (e.g. for Single Sign On) instance of the internal user store, and the scraper does not support this, failing to connect to the OpenEDX instance and hence stopping processing.

While it seems complex (infeasible ?) to support all kind of external identity providers, we have to consider :

• is it feasible to detect that an external identity provider is configured and fail with a nice message in such situations ?
• is it feasible to support some external identity providers with generic code ? (at least the most popular ones, probably Keycloak, Okta, ...)