fix: Unauthenticated BFF endpoints should still check logged-in JWT tokens

[pwnage101]

Tested against Alex's PR (#778) and the unmerged edx-drf-extensions changes (openedx/edx-drf-extensions#528).

Unauthenticated case:

$ pytest -vv --no-cov enterprise_access/apps/api/v1/tests/test_checkout_bff_views.py::CheckoutBFFViewSetTests::test_context_endpoint_unauthenticated_access
[...]
(Pdb) request.user
<django.contrib.auth.models.AnonymousUser object at 0x108d31fa0>

Authenticated case:

$ pytest -vv --no-cov enterprise_access/apps/api/v1/tests/test_checkout_bff_views.py::CheckoutBFFViewSetTests::test_context_endpoint_authenticated_access 
[...]
(Pdb) request.user
<User: [email protected]>

[pwnage101]

Merging blocked on first merging openedx/edx-drf-extensions#528, then bumping the edx-drf-extensions version locally.