Add daily_usage field to GitlabSubscriptions::User::CreditsUsageType

What does this MR do and why?

This adds daily_usage field to GitlabSubscriptions::User::CreditsUsageType

Users will be able to query their own GitLab Credits daily usage.

Relates to #605986 (closed)

References

• Issue: #605986 (closed)
• CustomersDot counterpart MR: https://gitlab.com/gitlab-org/customers-gitlab-com/-/merge_requests/17037
• Epic: gitlab-org#22817 (closed)
• UI prototype: https://product-rapidprototype-a1c47d.gitlab.io/self-scoped-credits-usage/
• Feature Planning issue: https://gitlab.com/gitlab-org/gitlab/-/work_items/604722

Screenshots or screen recordings

How to set up and validate locally

1. Ensure you're running local GDK in SAAS mode (e.g. export GITLAB_SIMULATE_SAAS=1 gdk restart)
2. Make sure you have Customers Dot running with the branch from this MR: https://gitlab.com/gitlab-org/customers-gitlab-com/-/merge_requests/17037
3. Enable user_gitlab_credits_dashboard
4. Have your user be part of one or more groups with gitlab credits
5. Consume some credits for your user
6. Execute this query on GraphQL explorer https://gdk.test:3443/-/graphql-explorer

query getUserCreditsUsage {
  currentUser {
    creditsUsage(namespacePath: "your-group") {
      creditsUsed
      dailyUsage {
        date
        creditsUsed
      }
    }
  }
}

MR acceptance checklist

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