Add usage billing dashboard API fetching

What does this MR do and why?

Adds API calling to Usage Billing Dashboard app, with loading and error state.

When error occurs we fallback to mock data, that is a placeholder until we have proper BE.

Adds Storybook stories for Commitment, No-commitment, loading and error states.

References

https://gitlab.com/gitlab-org/gitlab/-/issues/566202+

Screenshots or screen recordings

State	UI
Loaded
Loading
Error

How to set up and validate locally

To test it on GDK:

• Ensure you're running GitLab as self-managed (e.g. 'export GITLAB_SIMULATE_SAAS=0 gdk restart`)
• Ensure you've a license added to your instance
• Enable billing_usage feature flag (e.g. http://gdk.test:3000/rails/features/usage_billing_dev)
• Navigate to the /admin/gitlab_duo/usage
• Ensure the page is rendered with the Vue app

To test on storybook:

• Start storybook (e.g. yarn storybook:start)
• Open Storybook on http://localhost:6006/?path=/docs/ee-usage-quotas-usage-billing-app--docs

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.