Include VSD panels in the built-in visualization list

What does this MR do and why?

This MR adds Value Stream Analytics (VSA) visualization panels to the built-in visualization list in the Analytics Dashboard Editor. The change enables users to access and utilize VSA panels directly within the analytics dashboard, improving the integration of Value Stream metrics visualization capabilities.

Screenshots

Before	After - Group namespace	After - Project namespace

Key Changes and Actions

Change	Description
Feature Flag	Introduces vsd_visualizations_in_analytics_dashboard_editor feature flag
New Method	Adds vsd_dashboard_editor_enabled? helper method
Visualization List	Includes VSA panels in built-in visualizations when enabled
Testing	Updates specs to account for additional VSA visualizations

References

• Issue: #508776 (closed) - Value Stream Analytics panels in Analytics Dashboard Editor

MR acceptance checklist

• Feature flag implementation follows GitLab standards
• Test coverage added for new functionality
• Documentation updated to reflect new features
• Code follows established patterns for visualization integration

How to set up and validate locally

1. Enable the feature flag in rails console:

   Feature.enable(:vsd_visualizations_in_analytics_dashboard_editor)

2. Navigate to Analytics Dashboard Editor for a group or project
3. Verify that VSA visualization panels are available in the built-in visualizations list
4. Confirm that panels can be added to dashboards when the feature flag is enabled
5. Test that panels are not visible when the feature flag is disabled