Add AI flow triggers controller and initial views

What does this MR do and why?

This MR adds the controller and views for AI Flow Triggers functionality, enabling users to manage automated AI workflows within GitLab projects.

Related issue: https://gitlab.com/gitlab-org/gitlab/-/issues/559051+

Changes included:

Permissions & Policies:

• Extended ProjectPolicy with AI flow triggers permissions
• Added manage_ai_flow_triggers and trigger_ai_flow abilities
• Permissions require Duo Enterprise subscription and appropriate project access

Navigation:

• Updated Duo Agents sidebar menu to include Flow Triggers
• Added conditional menu item based on user permissions
• Maintained backward compatibility with existing agent sessions

Feature Flag:

• Added ai_flow_triggers feature flag (WIP, disabled by default)
• Scoped to Duo Workflow group for controlled rollout

Screenshots:

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