Support post processor on paste entities.

Summary:
this is to address the case where people paste content from quip
or google doc which breaks KM editor.

Differential Revision: D16833432

fbshipit-source-id: cc853284c833f131e11930cc4cf18714809071ac

Author: Tee Xie

src/component/base/DraftEditorProps.js

@@ -187,7 +187,7 @@ export type DraftEditorProps = {
 
   // Overrides for cut, copy & paste, which can be used to implement custom
   // behavior like entity cut/copy/paste (see PR #1784)."
-  onPaste?: (DraftEditor, SyntheticClipboardEvent<>) => void,
+  onPaste?: (DraftEditor, SyntheticClipboardEvent<>) => void | Promise<void>,
   onCut?: (DraftEditor, SyntheticClipboardEvent<>) => void,
   onCopy?: (DraftEditor, SyntheticClipboardEvent<>) => void,
 };