Draft: Add AI usage events aggregation engine

What does this MR do and why?

This changes lies the foundation for the feature retention metrics #580115

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

query SortByDateMonthly {
  group(fullPath: "flightjs") {
    aiUsage {
      events(
        feature: "code_suggestions"
        dateFrom: "2025-10-01"
        dateTo: "2026-02-28"
        orderBy: [{identifier: "eventDate", parameters: {granularity: "monthly"}, direction: ASC}]
      ) {
        nodes {
          dimensions {
            eventDate(granularity: "monthly")
            feature
          }
          lastPeriodUsersCount
          returningUsersCount
          usersCount
        }
      }
    }
  }
}

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Related to #580115