[TASK] Add support for having files outside the t3d/xml in impexp

Impexp files are blown up from the files that are included as base64.
This patch adds the possibility to have the files separated from the
 t3d/xml export file. They are saved to a folder beside the export file.

Resolves: #57379
Releases: 6.3, 6.2
Change-Id: Ib8bdc4336c64d2eae0e993132a5fce84eb01f766
Reviewed-on: https://review.typo3.org/30453
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring