Add Customers API to check whether customer should see overage alerts for the subscription usage

Problem

As part of gitlab#348481 (closed), we need to be able to determine whether customer should see overage alerts for the subscription usage the subscription has the reconciliation flag enabled on Zuora, so that we only show alerts for the appropriate subscriptions.

ATM a subscription will be charged for usage overage in several scenarios. This would change as we improve the process. If we won't(do not want to , or not ready to) charge the customer for a subscription's usage overage, we do not show the alert(otherwise it is confusing to customer).

This issue will iterate from show alert to subscription has the reconciliation flag enabled on Zuora, which I think it includes:

• Use case 3: Sale Assisted QSR in the table.
• Use case 6: self service QSR in the table.

Proposal

Create or modify an endpoint on the customers application to expose this information to the GitLab application and create the functionality on the GitLab application to fetch and cache this data.