[TASK] Add a signal after importer init in ImportExportUtility

To be able to modify importer options on importing T3D-files on
extension installation (e.g. for distributions) this patch introduces
a signal in ImportExportUtility.

Resolves: #58782
Releases: 6.3, 6.2
Change-Id: I9a3dec39efe4b8205013614cef1624720aab402a
Reviewed-on: https://review.typo3.org/30065
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn