Add IMS Authentication

Feature request.

### Expected Behaviour

Support authentication by virtue of IMS authentication (pre-existing token or full circle).
Stretch goal: Potentially support resource-specific entitlements.

### Actual Behaviour

Only OW credentials are supported.

Start by tracing the existing OW-credential authentication path and identify where IMS authentication can accept a pre-existing token or a full-circle flow. Done means IMS authentication is supported alongside OW credentials; resource-specific entitlements are explicitly a stretch goal and should be treated separately.

Written by the indexing model from the issue text.