Add sparklines to the analytics Exec Dashboard

Issue: #382063 (closed)

What does this MR do and why?

Adds a new column for sparkline charts to the analytics Exec Dashboard, which show data for the past 6 months starting from the current day. We only load the chart data once the table data has been successfully loaded, since the chart data is supplementary to the primary table data.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Seed cycle analytics data (and optionally DORA metrics data)
2. Enable the Exec dashboard

   Feature.enable(:group_analytics_dashboards_page)

3. Visit the exec dashboard: http://gdk.test:3000/groups/${YOUR_GROUP}/-/analytics/dashboards

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.