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/admin_dashboard_new_features_spec.rb

@@ -90,6 +90,28 @@ describe "Admin New Features Page", type: :system do
     expect(new_features_page).to have_no_screenshot
   end
 
+  it "displays experimental feature toggle" do
+    DiscourseUpdates.stubs(:new_features).returns(
+      [
+        {
+          "id" => 7,
+          "user_id" => 1,
+          "emoji" => "😍",
+          "title" => "New feature",
+          "description" => "New feature description",
+          "link" => "https://meta.discourse.org",
+          "tier" => [],
+          "discourse_version" => "",
+          "created_at" => "2023-11-10T02:52:41.462Z",
+          "updated_at" => "2023-11-10T04:28:47.020Z",
+          "experiment_setting" => "experimental_form_templates",
+        },
+      ],
+    )
+    new_features_page.visit
+    expect(new_features_page).to have_toggle_experiment_button
+  end
+
   it "displays a new feature indicator on the sidebar and clears it when navigating to what's new" do
     DiscourseUpdates.stubs(:has_unseen_features?).returns(true)
     visit "/admin"