[FEATURE] Add PSR-14 Event to modify loaded form definitions

This introduces the new PSR-14 AfterFormDefinitionLoadedEvent,
enabling extension authors to modify loaded form definitions.

The Event is dispatched by the `FormPersistenceManager` after
the form definition was loaded from either cache or the
filesystem, just before being overridden by TypoScript.

Resolves: #102865
Releases: main
Change-Id: Ic6c6490cd6259a81f1579a48c891d611552dd99d
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/82513


Reviewed-by: Nikita Hovratov <nikita.h@live.de>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Nikita Hovratov <nikita.h@live.de>
Tested-by: core-ci <typo3@b13.com>