Expose Ai::UsageEvent to GraphQL AiUsageData

Summary

As part of our effort to bring generalized AI usage data to production-ready state in scope of this issue we need to

Proposal

Expose Ai::UsageEvent model to GraphQL under AiUsageData field. PoC can be found in: !194207 (closed)

Technical requirements

1. expose id, user, event, timestamp fields only. Don't expose extras for now. Exposing extras requires security review.
2. inherit access rights from AiUsageData field.
3. suggested structure
4. The endpoint should be PG-only for now

aiUsageData {
  all(possible_filters) {
    nodes {
      id event timestamp user { username }
    }
  }
}

4. automatically pickup new event types, inherit event type enum from Ai::UsageEvent.events
5. Hide new endpoint behind unified_ai_events_graphql derisk feature flag.