Make Sidebar Extensions more idiomatic

Summary: Sidebar extensions were unkeyed, which make them render inefficiently. I cleaned the api a bit here by making the extensions more idiomatic; they are now components rather then functions, so that they have their own render cycle, state, etc. They are memo-ed now as well, so that they don't have to re-render if the selected item doesn't change.

Reviewed By: nikoant

Differential Revision: D27685980

fbshipit-source-id: b133bc42061b3b8cf971792f5818810ecb80e3ea

desktop/plugins/public/layout/extensions/fb-stubs/SidebarExtensions.tsx

@@ -0,0 +1,20 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @format
+ */
+
+import type {Client, Logger, PluginClient, Element} from 'flipper';
+
+export const SidebarExtensions: Record<
+  string,
+  React.FC<{
+    client: PluginClient;
+    realClient: Client;
+    selectedNode: Element;
+    logger: Logger;
+  }>
+> = {};