Add Duo assistance when creating fine-grained personal access tokens

What does this MR do and why?

Add DUO assistance for helping users select fine-grained permissions when creating personal access tokens.

References

Issue: #592230

Screenshots or screen recordings

How to set up and validate locally

1. Follow the GitLab Duo local development setup guide to configure your GDK with the AI gateway and Duo Chat enabled.
2. Check out the add-duo-permission-assistent-flow branch in gdk/gitlab-ai-gateway and do a gdk restart
3. Enable these feature flags in a Rails console:
   • Feature.enable(:granular_personal_access_tokens)
   • Feature.enable(:pat_permissions_suggestions)
4. Navigate to http://gdk.test:3000/-/user_settings/personal_access_tokens/granular/new and click "Set permissions with Duo".

MR acceptance checklist

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