Scaffold bulk update overrides modal behind a feature flag

What does this MR do and why?

• creates credit_caps_bulk_update Feature Flag
• passes that FF from BE to FE
• reads it to show or hide a button (the button will later show a bulk import modal)

References

• https://gitlab.com/gitlab-org/gitlab/-/work_items/630982+

Screenshots or screen recordings

Before (or FF disabled)	After

How to set up and validate locally

1. In rails console enable the feature flag

   Feature.enable(:credit_caps_bulk_update)

2. Visit admin area
3. Then GitLab Credits page
4. there click on Credit caps, leading to http://localhost:3000/admin/gitlab_credits_dashboard/credit_caps
5. See the presence of the button
6. Button does nothing (that's expected)

MR acceptance checklist

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