support createPaste

Summary:
Added support for `createPaste` in Sandy plugins

Nice minimalistic example of how to expose a Flipper api to Sandy.

Note that some indirection could be removed by having an interface that is shared directly between `BasePluginClient` and `FlipperLib` (e.g. `PublicFlipperLib`). In contrast to `addMenuEntries` from the previous diff, `createPaste` is basically exposed verbatim to Sandy without additional wrapping, so those cases could be made simpler. Maybe will do that later.

Reviewed By: passy

Differential Revision: D22815873

fbshipit-source-id: e6d0773a35341edfe5de0898317eaadf88de79d0

desktop/plugins/seamammals/src/index.tsx

@@ -59,8 +59,14 @@ export function plugin(client: FlipperClient<Events, {}>) {
     },
     {
       action: 'createPaste',
-      handler: () => {
-        console.log('creating paste');
+      handler: async () => {
+        const selection = selectedID.get();
+        if (selection) {
+          const url = await client.createPaste(
+            JSON.stringify(rows.get()[selection], null, 2),
+          );
+          alert(url); // TODO: use notifications T69990351
+        }
       },
     },
   );