[SECURITY] Validate allowed values for form element editors

Form editors which provide only a limited set of allowed values
(like single-select or multi-select form editors) now validate the
submitted values against the set of allowed values (configured within
the form setup).

Resolves: #93581
Releases: master, 11.1, 10.4, 9.5
Change-Id: Iae0a34c20cacdbcfc4eff9c4b1add966c1657010
Security-Bulletin: TYPO3-CORE-SA-2021-003
Security-References: CVE-2021-21357
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/68436


Tested-by: Oliver Hader <oliver.hader@typo3.org>
Reviewed-by: Oliver Hader <oliver.hader@typo3.org>