Add namespace settings around AI features

What does this MR do and why?

closes https://gitlab.com/gitlab-org/gitlab/-/issues/404856/

Design: https://gitlab.com/gitlab-org/gitlab/-/issues/404856/designs/MacBook_Pro_16__-_1.png

This MR adds two settings that are for top-level groups on SaaS only that are responsible for AI enablement for groups and project underneath.

Screenshots or screen recordings

How to set up and validate locally

1. Enable the main feature flag

   Feature.enable(:openai_experimentation)

2. Enable the secondary feature flag

   Feature.enable(:ai_related_settings)

3. Open GDK as SaaS (using GITLAB_SIMULATE_SAAS option)
4. In admin section, change plan of one group to ultimate.
5. In Group edit page, under Settings > General > Permissions and group features two new fields should appear. Experimental features should be disabled by default. Third-party AI features should be enabled by default.
6. After changing settings and saving new settings, the results should be persisted.

MR acceptance checklist

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