Introduce wallet agnostic view behind a feature flag

What does this MR do and why?

This creates a new feature flag to switch the GitLab Credits Dashboard view to a new one that we'll develop in https://gitlab.com/groups/gitlab-org/-/work_items/21464

References

• Epic: https://gitlab.com/groups/gitlab-org/-/work_items/21464+
• Issue: https://gitlab.com/gitlab-org/gitlab/-/work_items/594747+

Screenshots or screen recordings

For Group (SaaS)

FF off	FF on

For Instance (SM)

FF off	FF on

How to set up and validate locally

Turn on/off the wallet_agnostic_credits_dashboard FF: https://gdk.test:3443/rails/features/

1. For Group (SaaS)
   1. Make sure you're simulating SaaS
   2. Have a paid group
   3. Navigate to Group > Settings > GitLab Credits: https://gdk.test:3443/groups/<your-group>/-/settings/gitlab_credits_dashboard
   4. Ensure it behaves like the screenshots
2. For Instance (SM)
   1. Make sure you're NOT simulating SaaS
   2. Have a license applied to your instance
   3. Navigate to Admin > GitLab Credits: https://gdk.test:3443/admin/gitlab_credits_dashboard
   4. Ensure it behaves like the screenshots

MR acceptance checklist

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