Group setting toggle for hierarchy optimization

What does this MR do and why?

This MR adds a temporary group setting toggle (behind feature flag) for enabling the group hierarchy query optimizations for a group. I decided to have an actual toggle on the UI so we have an easy way to enable and disable the optimization for a group on production before rolling the optimization out globally. The other approach would be creating the Namespaces::Descendants record via rails console which make testing the feature on smaller/isolated groups difficult (creating a change request).

Related epic: &11469

MR acceptance checklist

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Screenshots or screen recordings

How to set up and validate locally

1. In rails console enable the experiment fully

   Feature.enable(:group_hierarchy_optimization)

2. Go to a group and go to Settings > General
3. Find the performance toggle (bottom) in the Permissions and group features section.
4. The setting should be persisted.
5. When uncheckng the toggle, the setting should be also persisted (unchecked).

Related to #433478 (closed)