[FEATURE] Add getValidators to AbstractCompositeValidator

removeValidator is unusable as long as there is no method
to get contained validators

Resolves: #51707
Releases: 6.2, 6.1, 6.0
Change-Id: If8705e8b5c7c2dfa84a732b97813d6097930f209
Reviewed-on: https://review.typo3.org/23573
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind