Update Duo Workflow with auth
What does this MR do and why?
- Add more color on how Duo Workflow executor will authenticate to the Duo Workflow Service and how the Duo Workflow Service will authenticate to GitLab Rails.
- This plan has changed slightly now that we are building the Duo Workflow Service in its own codebase. Before, it was part of the AI Gateway codebase and as a result the authentication mechanism more closely mimicked how Code Completion requests are authenticating directly to the AI Gateway.
- This new plan also explains how the Duo Workflow Service will know which GitLab Rails instance to connect to for API requests.
- Also clarify that we are now building the Duo Workflow Service in its own codebase.
- Addresses conversation here: gitlab-org/modelops/applied-ml/code-suggestions/ai-assist#527 (comment 1979632865)
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