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.

app/controllers/admin/dashboard_controller.rb

@@ -48,6 +48,18 @@ class Admin::DashboardController < Admin::StaffController
     render json: data
   end
 
+  def toggle_feature
+    Experiments::Toggle.call(service_params) do
+      on_success { render(json: success_json) }
+      on_failure { render(json: failed_json, status: 422) }
+      on_failed_policy(:current_user_is_admin) { raise Discourse::InvalidAccess }
+      on_failed_policy(:setting_is_available) { raise Discourse::InvalidAccess }
+      on_failed_contract do |contract|
+        render(json: failed_json.merge(errors: contract.errors.full_messages), status: 400)
+      end
+    end
+  end
+
   private
 
   def mark_new_features_as_seen