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

@@ -328,3 +328,21 @@ test('plugins can register menu entries', async () => {
     `"No menu entry found with action: Non Existing"`,
   );
 });
+
+test('plugins can create pastes', async () => {
+  const plugin = TestUtils.startPlugin({
+    plugin(client: PluginClient) {
+      return {
+        doIt() {
+          client.createPaste('test');
+        },
+      };
+    },
+    Component() {
+      return null;
+    },
+  });
+
+  plugin.instance.doIt();
+  expect(plugin.flipperLib.createPaste).toBeCalledWith('test');
+});