Feature flag and scaffolding for visualization designer filter overhaul

What does this MR do and why?

Adds a new development feature flag analytics_visualization_designer_filtering.

When enabled, the product analytics visualization designer UI changes:

• existing measure and dimension selection components are hidden
• new stub filtered search component is displayed
  • contains an initial "measure" token
• preview component fills full width of container

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Before	After
	(flag enabled)
	(selecting a measure)
	(flag disabled, note at mobile the preview pane now wraps under the selectors)

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. Visit the visualization designer at Project > Analytics > Analytics dashboards > Visualization designer.
4. Verify existing UI works as expected
5. Enable the new feature flag

• echo "Feature.enable(:analytics_visualization_designer_filtering)" | gdk rails c

6. Verify old metric/dimension selection UI is hidden, new stub filter bar is displayed
7. Selecting a measure is possible and visualization can be saved

Related to #462614 (closed)