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/controllers/admin/plugins_controller.rb

@@ -8,4 +8,16 @@ class Admin::PluginsController < Admin::StaffController
       root: "plugins",
     )
   end
+
+  def show
+    plugin = Discourse.plugins_by_name[params[:plugin_id]]
+
+    # An escape hatch in case a plugin is using an un-prefixed
+    # version of their plugin name for a route.
+    plugin = Discourse.plugins_by_name["discourse-#{params[:plugin_id]}"] if !plugin
+
+    raise Discourse::NotFound if !plugin
+
+    render_serialized(plugin, AdminPluginSerializer, root: nil)
+  end
 end