Add Admin Settings for Namespace Storage Cost Factor for Forks

What does this MR do and why?

Add a setting for namespace storage cost factor for forks to the admin backend UI.

The setting does not appear in any menus. For instance, it does not have a link in the sidebar. It is available directly at the /admin/application_settings/namespace_storage path.

Issue: https://gitlab.com/gitlab-org/gitlab/-/issues/404628

Screenshots or screen recordings

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