[FEATURE] EXT:form - make FinisherVariableProvider iterable

Makes it possible to iterate through FinisherVariableProvider items.

Resolves: #83396
Releases: master
Change-Id: I0b778b9a6d4ede4d2df8190d6b4821903caee6a4
Reviewed-on: https://review.typo3.org/55308


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Joerg Boesche <typo3@joergboesche.de>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>