Add duo_workflow_prompt_registry feature flag

What does this MR do and why?

Introduces the feature flag from gitlab-org/modelops/applied-ml/code-suggestions/ai-assist!2893 (merged) (see specifically here)

Part of gitlab-org/modelops/applied-ml/code-suggestions/ai-assist#1072 (closed)

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

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