Use mock server for usage quota check in dev

What does this MR do and why?

Use mock server for usage quota check in dev

Use a mock server url https://gitlab.com/gitlab-org/ai-powered/mock-cred-cd when in development env duo to complexity of dealing with Customers dot portal.

References

https://gitlab.com/gitlab-org/gitlab/-/work_items/582179+

How to set up and validate locally

1. Run the mock server
2. In a rails console attempt to connect to the server with Gitlab::SubscriptionPortal::Client.verify_usage_quota(user_id: ..., root_namespace_id: ...)
3. Use Feature.enable(:use_mock_dot_api_for_usage_quota) or disable to enable and disable the flag
4. When enabled it should contact the mock server, and when disabled it should contact the staging server

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