Implement VSD GraphQL query

What does this MR do and why?

This MR implements the value stream dashboard overview count aggregation query and exposes the data via GraphQL.

GraphQL query:

{
  group(fullPath: "gitlab-org") {
    valueStreamDashboardUsageOverview(identifier: PROJECTS, timeframe: { start: "2021-01-01", end: "2023-08-31" }) {
      identifier
      count
      recordedAt
    }
  }
}

Database

Query with data (produced by invoking Analytics::ValueStreamDashboard::CountWorker with PG.ai):

• Cached: https://explain.depesz.com/s/RsH8
• Uncached (no aggregations data on PRD yet, the query mostly reads the group hierarchy): https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/21536/commands/70081

How to verify

1. Ensure that you're on ultimate.
2. Find the gitlab-org group and create some test counts.

group = Group.find_by_path("gitlab-org")
FactoryBot.create(:value_stream_dashboard_count, namespace: group, count: 500, recorded_at: "2023-08-20")

1. Invoke the graphql query mentioned above

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #402997 (closed)