Support API gateway based authentication

Description

Instead of direct ibis access support access via API gateway. This crucially removes lookup/ibis specific credentials from deployments.

Ideally support this via the current function interface, such that a major release change is not needed. This may help remove ibis creds from existing deployments.

One potential avenue for this is via the openAPI generate identity client: https://gitlab.developers.cam.ac.uk/uis/devops/iam/identity-lib/

The documentation between the two repos could also then potentially be co-oridinated to guide on correct usage (e.g. use ucamlookup for django only functionality like creating users and use identity-lib for all other purposes).