swh.auth.django: Expose OIDCUser model object

That class is a custom User proxy model for remote users storing OpenID Connect related data (profile containing authentication tokens, ...).

The model is also not saved to database as all users are already stored in the Keycloak one.

That class will be used for example by both the webapp and the deposit.

No test yet, just the plain minimal django test configuration for the tests to not break...

Related to T3079

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Migrated from D5219 (view on Phabricator)