[VSA] Expose duration data series via GraphQL

Extend the valueStreams.stages.metrics GraphQL node with a new node: series. For the series node, specify a new node averageDurations. The new node can be used to replace the average durations chart (time series data) private API endpoint (example: https://gitlab.com/groups/gitlab-org/-/analytics/value_stream_analytics/value_streams/1006837/stages/1032713/average_duration_chart?created_after=2024-07-11&created_before=2024-08-09)

Example GraphQL query:

group(fullPath: "gitlab-org") { // or project
  valueStreamAnalytics {
    valueStreams(id: '') {
      stages(id: '') {
        metrics(assignee_username: ...) {
          median { // implemented
            value
            title
          }
          average { // implemented
          }
          count { // implemented
          }
          // TO BE IMPLEMENTED
          series {
            averageDurations { // array of Types::Analytics::CycleAnalytics::DateMetricType inherit from  Types::Analytics::CycleAnalytics::DateMetricType
              nodes {
                date // new field
                value
              }
            }
          }
        }
      }
    }
  }
}