[TASK] Cleanup EXT:impexp

This patch serves as a basis for further improving
the UX and new features for import / export.

* Better separation between controllers and worker services
* Refactor code & streamline wording
* More strict typing and some PHP 8 related fixes
* Streamline exception handling
* Support full import configuration in CLI command impexp:import
* Functional tests with increased coverage and less mocking
* A set of acceptance tests for the backend interface
* Minor bug fixes

Note: The output of the extension has not been changed
in any way, e.g. the format of the export file remains
the same. Only the internal processes have been optimized.

Resolves: #94772
Releases: master
Change-Id: I4df66a6f615e07d6d9767723a12a769aedeb341b
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/70332


Tested-by: core-ci <typo3@b13.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Benni Mack <benni@typo3.org>