(General) Callback support for export methods

[Synthoid]

It would be nice to allow callbacks to be passed to the JSON and XML export methods. This would allow users to create methods that can handle exported data directly.

[Synthoid]

This will be implemented as of V60. Users will be able to pass callbacks to exportSpreadsheetXml and exportSpreadsheetJson methods to handle exported data strings. See release notes for what is expected in callback methods.