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/flipper-plugin/src/test-utils/test-utils.tsx

@@ -100,6 +100,8 @@ interface StartPluginResult<Module extends FlipperPluginModule<any>> {
     }[],
   ): void;
 
+  triggerDeepLink(deeplink: unknown): void;
+
   exportState(): any;
 }
 
@@ -164,6 +166,9 @@ export function startPlugin<Module extends FlipperPluginModule<any>>(
       });
     },
     exportState: () => pluginInstance.exportState(),
+    triggerDeepLink: (deepLink: unknown) => {
+      pluginInstance.triggerDeepLink(deepLink);
+    },
   };
   // @ts-ignore
   res._backingInstance = pluginInstance;