DEV: Single admin plugin page for consistent admin plugin UX (#26024)

This commit adds new plugin show routes (`/admin/plugins/:plugin_id`) as we move
towards every plugin having a consistent UI/landing page.

As part of this, we are introducing a consistent way for plugins
to show an inner sidebar in their config page, via a new plugin
API `register_admin_config_nav_routes`

This accepts an array of links with a label/text, and an
ember route. Once this commit is merged we can start the process
of conforming other plugins to follow this pattern, as well
as supporting a single-page version of this for simpler plugins
that don't require an inner sidebar.

Part of /t/122841 internally

plugins/chat/spec/requests/application_controller_spec.rb

@@ -22,7 +22,7 @@ RSpec.describe ApplicationController do
       sign_in(admin)
       get "/latest"
       expect(JSON.parse(preloaded_json["enabledPluginAdminRoutes"])).to include(
-        { "label" => "chat.admin.title", "location" => "chat" },
+        { "label" => "chat.admin.title", "location" => "chat", "use_new_show_route" => false },
       )
     end
   end