Make plugin loading async

Summary: This diff makes plugin loading async, which we'd need in a browser env (either because we'd use `import()` or we need to fetch the source and than eval it), and deals with all the fallout of that

Reviewed By: timur-valiev

Differential Revision: D32669995

fbshipit-source-id: 73babf38a6757c451b8200c3b320409f127b8b5b

desktop/flipper-ui-core/src/RenderHost.tsx

@@ -98,6 +98,7 @@ export interface RenderHost {
   GK(gatekeeper: string): boolean;
   flipperServer: FlipperServer;
   serverConfig: FlipperServerConfig;
+  requirePlugin(path: string): Promise<any>;
 }
 
 export function getRenderHostInstance(): RenderHost {