Add custom_dashboard_storage feature flag

Summary

Add a feature flag for the custom dashboards storage feature to enable controlled rollout and provide a kill switch if issues arise.

Background

As part of the Custom Dashboards Foundation initiative (Epic #19431), we're implementing database-backed storage for custom dashboards. This moves away from repository file storage to enable:

• Dashboard creation at organization and namespace levels
• Fast database-backed search across dashboards
• Centralized dashboard management
• Dashboards independent of project repositories