[TASK] Flexform support for type:passthrough

Resolves: #20705
Releases: master
Change-Id: I3476f6945b645a3b906e49efbdeaa385411c50ea
Reviewed-on: https://review.typo3.org/47491
Tested-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Tested-by: Frank Naegler <frank.naegler@typo3.org>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>