FEATURE: More flexible admin plugin config nav definition (#26254)

This commit changes the API for registering the plugin config
page nav configuration from a server-side to a JS one;
there is no need for it to be server-side.

It also makes some changes to allow for 2 different ways of displaying
navigation for plugin pages, depending on complexity:

* TOP - This is the best mode for simple plugins without a lot of different
  custom configuration pages, and it reuses the grey horizontal nav bar
  already used for admins.
* SIDEBAR - This is better for more complex plugins; likely this won't
  be used in the near future, but it's readily available if needed

There is a new AdminPluginConfigNavManager service too to manage which
plugin the admin is actively viewing, otherwise we would have trouble
hiding the main plugin nav for admins when viewing a single plugin.

spec/lib/plugin/instance_spec.rb

@@ -999,30 +999,4 @@ TEXT
       expect(sum).to eq(3)
     end
   end
-
-  describe "#register_admin_config_nav_routes" do
-    let(:plugin) { Plugin::Instance.new }
-
-    it "adds the specified plugin id as the 'model' for the route" do
-      plugin.register_admin_config_nav_routes(
-        "discourse-awesome",
-        [{ route: "adminPlugins.show", label: "some.i18n.label" }],
-      )
-      expect(plugin.admin_config_nav_routes).to eq(
-        [{ route: "adminPlugins.show", label: "some.i18n.label", model: "discourse-awesome" }],
-      )
-    end
-
-    it "errors if the route or label is not provided" do
-      expect {
-        plugin.register_admin_config_nav_routes("discourse-awesome", [{ label: "some.i18n.label" }])
-      }.to raise_error(ArgumentError)
-      expect {
-        plugin.register_admin_config_nav_routes(
-          "discourse-awesome",
-          [{ route: "adminPlugins.show" }],
-        )
-      }.to raise_error(ArgumentError)
-    end
-  end
 end