Read the "third-party AI by default" setting when authenticating users

This MR allows us to read the token sent by the code suggestion clients. This token decides whether we serve the codegen or the text-bison model.

Relates to https://gitlab.com/gitlab-org/modelops/applied-ml/code-suggestions/ai-assist/-/issues/142 and https://gitlab.com/gitlab-org/modelops/applied-ml/code-suggestions/ai-assist/-/issues/143

Blocked by: !153 (merged)