ExportCallback Object

User-defined callback function used with win32file::ReadEncryptedFileRaw.
Function is called with 3 parameters: (Data, CallbackContext, Length)
   Data: Read-only buffer containing the raw data read from the file. Must not be referenced outside of the callback function.
   CallbackContext: Arbitrary object passed to ReadEncryptedFileRaw.
   Length: Number of bytes in the Data buffer.
On success, function should return ERROR_SUCCESS. Otherwise, it can return a win32 error code, or simply raise an exception.