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

app/serializers/admin_plugin_serializer.rb

@@ -16,7 +16,12 @@ class AdminPluginSerializer < ApplicationSerializer
              :commit_hash,
              :commit_url,
              :meta_url,
-             :authors
+             :authors,
+             :admin_config_nav_routes
+
+  def admin_config_nav_routes
+    object.admin_config_nav_routes
+  end
 
   def id
     object.directory_name
@@ -67,7 +72,12 @@ class AdminPluginSerializer < ApplicationSerializer
     return unless route
 
     ret = route.slice(:location, :label)
-    ret[:full_location] = "adminPlugins.#{ret[:location]}"
+    if route[:use_new_show_route]
+      ret[:full_location] = "adminPlugins.show.#{ret[:location]}"
+      ret[:use_new_show_route] = true
+    else
+      ret[:full_location] = "adminPlugins.#{ret[:location]}"
+    end
     ret
   end