Add Restricted Access Option to Seat Controls

What does this MR do and why?

Add Restricted access option to seat controls.

Choosing restricted access turns on block seat overages for the group.

This new option is behind a feature flag.

Issue: https://gitlab.com/gitlab-org/gitlab/-/issues/468498

MR acceptance checklist

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Screenshots

Before	After

When the feature cannot be enabled because an external group has been invited

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.