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

@@ -9,11 +9,12 @@
 
 import type {Client, Logger, PluginClient, Element} from 'flipper';
 
-export default [] as Array<
-  (
-    client: PluginClient,
-    realClient: Client,
-    selectedNode: Element,
-    logger: Logger,
-  ) => React.ReactNode
->;
+export const SidebarExtensions: Record<
+  string,
+  React.FC<{
+    client: PluginClient;
+    realClient: Client;
+    selectedNode: Element;
+    logger: Logger;
+  }>
+> = {};