Support handling deeplinks in plugins

Summary:
This adds support for handling incoming deeplinks in a Sandy plugin, which can be done by using a `client.onDeepLink(deepLink => { } )` listener

Also generalized deeplinks to not just support strings, but also richer objects, which is beneficial to plugin to plugin linking.

Reviewed By: jknoxville

Differential Revision: D22524749

fbshipit-source-id: 2cbe8d52f6eac91a1c1c8c8494706952920b9181

desktop/app/src/plugin.tsx

@@ -89,8 +89,8 @@ export type Props<T> = {
   persistedState: T;
   setPersistedState: (state: Partial<T>) => void;
   target: PluginTarget;
-  deepLinkPayload: string | null;
-  selectPlugin: (pluginID: string, deepLinkPayload: string | null) => boolean;
+  deepLinkPayload: unknown;
+  selectPlugin: (pluginID: string, deepLinkPayload: unknown) => boolean;
   isArchivedDevice: boolean;
   selectedApp: string | null;
   setStaticView: (payload: StaticView) => void;