Add tracking for visualization designer

What does this MR do and why?

Adds user event tracking for the analytics dashboard designer for Add user tracking for visualization designer (#420826 - closed).

The following internally tracked events are added:

Description	Event name	Tracking location
User saves visualization	user_created_custom_visualization	frontend
User views visualization designer	user_viewed_visualization_designer	frontend

Note that analytics dashboards is an experimental feature hidden behind default off feature flags.

Screenshots or screen recordings

No visual change.

How to set up and validate locally

1. Follow the instructions to debug internal events tracking.
2. Enable custom dashboards on your GDK:
   • echo "Feature.enable(:combined_analytics_dashboards)" | gdk rails c
   • echo "Feature.enable(:combined_analytics_dashboards_editor)" | gdk rails c
   • echo "Feature.enable(:combined_analytics_visualization_editor)" | gdk rails c
3. View the dashboards list at Project > Analytics > Analytics dashboards and click on Configure Dashboard Project.
4. Select any repository to store your custom dashboard files.
5. View the dashboards list at Project > Analytics > Analytics dashboards.
6. Click on Visualization Designer button. It should track the user_viewed_visualization_designer event.
7. Create and save your new visualization. It should track the user_created_custom_visualization event.

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 #420826 (closed)