[TASK] FormEngine: Move flex form TCA migration to data provider

Move the TCA flex form migration from the FlexFormElementContainer
to the TcaFlex data provider to separate parsing and rendering.

Resolves: #69718
Releases: master
Change-Id: Id9e132483394ff8efac0f21441798ff469ae3091
Reviewed-on: http://review.typo3.org/43276


Reviewed-by: Nicole Cordes <typo3@cordes.co>
Tested-by: Nicole Cordes <typo3@cordes.co>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>