Draft: Expose `GitlabSubscriptionHistory` via GraphQL

What does this MR do and why?

This is part of https://gitlab.com/groups/gitlab-org/-/epics/14290. We're implementing https://gitlab.com/gitlab-org/gitlab/-/issues/355985, exposing GitlabSubscriptionHistory via GraphQL so that we can show this data for users.

MR acceptance checklist

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

Screenshots or screen recordings

N/A

How to set up and validate locally

1. Switch to this MR's branch
2. Open GraphQL Explorer: https://gdk.test:3443/-/graphql-explorer
3. Run the following query:

    query getSubscriptionHistory($fullPath: ID!) {
      namespace(fullPath: $fullPath) {
        id
        subscriptionHistory {
          createdAt
          startDate
          endDate
          seats
          seatsInUse
          maxSeatsUsed
          changeType
      }
      }
    }