FEATURE: allow admins to enable announced experimental features (#29244)

Toggle the button to enable the experimental site setting from "What's new" announcement.

The toggle button is displayed when:
- site setting exists and is boolean;
- potentially required plugin is enabled.

spec/system/page_objects/pages/admin_new_features.rb

@@ -16,6 +16,10 @@ module PageObjects
         page.has_no_css?(".admin-new-feature-item__screenshot")
       end
 
+      def has_toggle_experiment_button?
+        page.has_css?(".admin-new-feature-item__feature-toggle")
+      end
+
       def has_learn_more_link?
         page.has_css?(".admin-new-feature-item__learn-more")
       end