Add credits_used field to subscriptionUsage

What does this MR do and why?

This adds the credits_used field to subscriptionUsage as part of https://gitlab.com/gitlab-org/gitlab/-/work_items/594756

Clients can now query for the total GitLab Credits used in their subscription.

This MR is dependent on https://gitlab.com/gitlab-org/customers-gitlab-com/-/merge_requests/15277+

References

• Issue: https://gitlab.com/gitlab-org/gitlab/-/work_items/594756+

Screenshots or screen recordings

How to set up and validate locally

1. Add some transactions to your subscription's wallet (docs)
2. Open GraphQL and execute this query (https://gdk.test:3443/-/graphql-explorer) :

Query:

query getSubscriptionUsage($namespacePath: ID) {
  subscriptionUsage(namespacePath: $namespacePath) {
    creditsUsed
  }
}

Variables:

{
  "namespacePath": "your-group"
}

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