[VSA] Add GraphQL item pagination

This issue is for implementing the paginated list of related-items for VSA GraphQL API.

Let's add an items paginated node within the metrics node since all filters should also apply:

group(fullPath: "gitlab-org") { // or project
  valueStreamAnalytics {
    valueStreams(id: '') {
      // already implemented fields
      stages(id: '') {
        // already implemented fields
        metrics(assignee_username: ...) { 
          items(pagination arguments:)  {
            nodes {
              duration // provided by the aggregation query
              startEventTimestamp // provided by the aggregation query
              endEventTimestamp // provided by the aggregation query
              title // issues.title or merge_request.title
              // see notes below
            }
            pageInfo {
              startCursor
            }
          }
        }
      }
    }
  }
}

Notes

• To provide the list of items we can call the serialized_records method on the DataCollector. We can reuse the invocation logic from #410327 (closed) (requiest params -> data collector)
• To decide: use the serialized_records method or expose the actual issue or MR types? In this case we probably needs some sort of union type in GraphQL. To be discussed.
• At start, we can use offset pagination. Reasoning: the group-level query uses the in-operator optimization which cannot be keyset paginated at the moment (can be done but requires some work).
• Use the default sorting: end event DESC