Add use_billing_staging_endpoint feature flag

What does this MR do and why?

This MR introduces a temporary feature flag duo_use_billing_endpoint that controls whether usage billing events are routed to the staging billing endpoint instead of production for testing purposes

References

https://gitlab.com/gitlab-org/architecture/usage-billing/design-doc/-/issues/32

Screenshots or screen recordings

N/A

How to set up and validate locally

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