Add enabled field to the SubscriptionUsage API

What does this MR do and why?

Related to #580097

This MR adds a new Boolean field, enabled, to the Subscription Usage GraphQL API. It proxies the value of the same name from the Subscription Portal (CDot) GraphQL API. This field will be used by the frontend to show a disabled banner on the usage dashboards.

References

Issue: https://gitlab.com/gitlab-org/gitlab/-/issues/580097

Screenshots or screen recordings

How to set up and validate locally

1. Pull the latest from the main branch in CDot

2. On GitLab, open GraphQL explorer https://gdk.test:3443/-/graphql-explorer

3. Execute the query below (while replacing the namespacePath variable with your local group path)

   Query

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

   Variables

   {
     "namespacePath": "full_path_of_your_group"
   }

4. You should see the enabled field returned with a value of true.

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