[TASK] EXT:form - add onSubmit callback for pages

Add onSubmit callback for toplevel composit elements aka. pages.
The ticket provides an example extension for testing.

Resolves: #78821
Releases: master
Change-Id: Ie521ba75472e426594dae231ee86f68cd241f23d
Reviewed-on: https://review.typo3.org/50796


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Bjoern Jacob <bjoern.jacob@tritum.de>
Tested-by: Bjoern Jacob <bjoern.jacob@tritum.de>
Reviewed-by: Daniel Lorenz <daniel.lorenz@extco.de>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>

typo3/sysext/form/Classes/Domain/Runtime/FormRuntime.php

@@ -404,6 +404,9 @@ class FormRuntime implements RootRenderableInterface, \ArrayAccess
                 $propertyPathsForWhichPropertyMappingShouldHappen[$temporaryPropertyPath] = $temporaryPropertyPath;
             }
         };
+
+        $value = null;
+        $page->onSubmit($this, $value, $requestArguments);
         foreach ($page->getElementsRecursively() as $element) {
             try {
                 $value = ArrayUtility::getValueByPath($requestArguments, $element->getIdentifier(), '.');