Add Value Stream Dashboard setting

What does this MR do and why?

This MR adds a group setting to enable/disable the overview counts aggregation for value streams dashboard. The checkbox is behind a feature flag because the aggregation logic is not implemented yet.

Screenshots or screen recordings

How to set up and validate locally

1. Ensure you're on ultimate license
2. In rails console enable the FF: modify_value_stream_dashboard_settings

   Feature.enable(:value_stream_dashboard_on_off_setting)

3. Visit any top-level group > General settings > Analytics
4. Value Streams Dashboard section should have a checkbox

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 #402243 (closed)