Initial DataSource setup

Summary:
For context see https://fb.workplace.com/notes/470523670998369

This diff introduces the DataSource abstraction, that can store records. If a key is set a key -> record mapping is stored, to make it easy to update existing records using `upsert`, without knowing their exact index.

Internal storage will be slightly altered in upcoming diffs, so don't pay to much attention to that part.

Reviewed By: nikoant

Differential Revision: D25953337

fbshipit-source-id: 1c3b53a2fcf61abaf061946be4af21d2aecc6c6d

docs/extending/flipper-plugin.mdx

@@ -438,6 +438,10 @@ rows.update(draft => {
 console.log(rows.get().length) // 2
 ```
 
+### createDataSource
+
+Coming soon.
+
 ## React Hooks
 
 ### usePlugin