NativePlugins: Handle sidebar sections
NativePlugins: Handle sidebar sections

Summary:
Adds ability to send sidebar sections from the mobile app.
Currently implemented sections are json blob, and toolbar (row of buttons).

Note that I have not tested the toolbar with more than one button. I'm currently implementing features as needed so this should be considered alpha support at the moment.

Reviewed By: passy

Differential Revision: D14642879

fbshipit-source-id: 84876935337741422242285dc48da79af3a1d5ee

facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper