Add duo_workflow_work_item_tools feature flag

What does this MR do and why?

This MR adds a feature flag to be used in Duo Workflow Service to manage the rollout of work item tools.

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. In rails console, enable duo_workflow_work_item_tools feature flag

   Feature.enable(:duo_workflow_work_item_tools)
   workflow_id = Ai::DuoWorkflows::Workflow.last

2. Make a head request to workflow endpoint curl --head -I http://gdk.test:3000/api/v4/ai/duo_workflows/workflows/workflow_id\?private_token\=$GDK_TOKEN

3. You should see duo_workflow_work_item_tools in the X-Gitlab-Enabled-Feature-Flags header

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to gitlab-org/modelops/applied-ml/code-suggestions/ai-assist#1145 (closed)