Add admin usage billing route

What does this MR do and why?

For more details please check the issue: https://gitlab.com/gitlab-org/gitlab/-/issues/566198

Description	MR
Route for SM	👈 you're here
Route for .com	!203266 (merged)

Feature flag was named usage_billing_dev to match the same naming on CustomerDot and to avoid clashing with the License Feature name

Screenshots or screen recordings

N/A

How to set up and validate locally

1. Have a valid license in your GDK
2. Access the url: https://gdk.test:3443/admin/gitlab_duo/usage
3. Check if it loads:
   1. If GITLAB_SIMULATE_SAAS=1 -> 404
   2. If GITLAB_SIMULATE_SAAS=0 -> OK
   3. If you remove your license -> 404
   4. If usage_billing_dev is enabled -> OK
   5. If usage_billing_dev is disabled -> 404

MR acceptance checklist

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