Move server add-on interfaces to flipper-common

Reviewed By: antonk52

Differential Revision: D34169419

fbshipit-source-id: 69fcc8da7d98f59818fbb5b486a488f19830cd81

desktop/flipper-common/src/index.tsx

@@ -51,3 +51,4 @@ export * from './clientUtils';
 export * from './settings';
 export * from './PluginDetails';
 export * from './doctor';
+export * from './ServerAddOn';