Add group usage billing route

What does this MR do and why?

For more details please check the issue: https://gitlab.com/gitlab-org/gitlab/-/issues/566199

Description	MR
Route for SM	!203257 (merged)
Route for .com	👈 you're here

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. Access the url: https://gdk.test:3443/groups/<your_group>/-/settings/gitlab_duo/usage
2. Check if it loads:
   1. If GITLAB_SIMULATE_SAAS=0 -> 404
   2. If GITLAB_SIMULATE_SAAS=1 -> OK
   3. If you remove your license -> 404
   4. If usage_billing_dev is enabled -> OK
   5. If usage_billing_dev is disabled -> 404
   6. If you're a group owner -> OK
   7. If your access level is lower than owner -> 404

MR acceptance checklist

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