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/test-utils/test-utils.tsx

@@ -324,6 +324,7 @@ export function renderDevicePlugin<Module extends FlipperDevicePluginModule>(
 export function createMockFlipperLib(): FlipperLib {
   return {
     enableMenuEntries: jest.fn(),
+    createPaste: jest.fn(),
   };
 }