Add "Group > Automate> Flows" menu item, routes, boilerplate code

What does this MR do and why?

• Add menu item, route, permissions, controller for Group > Automate > Flows.
• Add basic frontend code to render the Duo Agent Platform app for Flows.

Note: More changes will be added in later MRs.

References

Related to #578085

Screenshots or screen recordings

How to set up and validate locally

1. Prerequisite: You have a Duo workflow-enabled project.
2. Enable the feature flags global_ai_catalog, and ai_catalog_flows.
3. Go to Group > Automate > Flows. For example, https://gdk.test:3443/groups/gitlab-duo/-/automate/flows.

MR acceptance checklist

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