Inject nested components into parent component initialization in admin

By following this pattern, overriding the parent component becomes
easier, as only the initializer is needed.

That also allows to avoid slots and its problem with deeply nested
structures [1].

[1] - ViewComponent/view_component#867

admin/app/components/solidus_admin/main_nav_component.rb

@@ -3,14 +3,25 @@
 module SolidusAdmin
   # Renders the main navigation of Solidus Admin.
   class MainNavComponent < BaseComponent
+    def initialize(main_nav_item_component: component("main_nav_item"), items: container.within_namespace("main_nav"))
+      @main_nav_item_component = main_nav_item_component
+      @items = items
+    end
+
     erb_template <<~ERB
       <nav>
         <%=
-          render component("main_nav_item").with_collection(
-            container.all("main_nav").sort_by(&:position)
+          render @main_nav_item_component.with_collection(
+            sorted_items
           )
         %>
       </nav>
     ERB
+
+    private
+
+    def sorted_items
+      @items.sort_by(&:position)
+    end
   end
 end