Add analytics visualization designer feature flag

What does this MR do and why?

Add analytics visualization designer feature flag for [Feature flag] Rollout of `combined_analytics_v... (#425048 - closed).

Why? We need to decouple the visualization designer from the dashboard designer so that we can release the latter as part of our MVC. The former is not ready for release yet.

Changes made:

• Add a new combined_analytics_visualization_editor feature flag
• Hide the analytics "Visualization designer" button unless enabled
• Update specs

There is a separate MR to update the GDK docs Add visualization editor flag to product analyt... (gitlab-development-kit!3355 - merged).

Screenshots or screen recordings

Before	After (disabled)	After (enabled)

How to set up and validate locally

1. Follow these instructions to setup Product Analytics in GDK.
2. Onboard Product Analytics by setting up the feature at Project > Analytics > Analytics dashboards.
3. Enable custom dashboards by following the alert prompt on Project > Analytics > Analytics dashboards.
4. View the dashboard listing at Project > Analytics > Analytics dashboards.
5. Verify that New dashboard button is rendered.
6. Verify that the Visualization designer button is not rendered.
7. Enabled the feature flag by running echo "Feature.enable(:combined_analytics_visualization_editor)" | gdk rails c.
8. Reload the page and verify that the Visualization designer button is now rendered.

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 #425048 (closed)