Add usage billing app skeleton with its story

What does this MR do and why?

It adds a skeleton app for the usage billing page (which will be added behind a feature flag).

MR	status
Add usage billing app skeleton with its story (!203124 - merged)	this MR
Add formatted-date-range component to usage_bil... (!203275 - closed)	⏳
Add purchase commitment card and use it in the ... (!203278 - merged)	⏳

References

Related to #566202

Screenshots or screen recordings

GDK	Storybook

How to set up and validate locally

• Checkout the branch
• Start storybook (e.g. yarn storybook:start)
• Navigate to http://localhost:6006/?path=/story/ee-usage-billing-app--default

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

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.

Related to #566202