Add BeforeFormValidateEvent for new and edit actions

[Amoifr]

Summary

Add a new BeforeFormValidateEvent dispatched after $form->handleRequest() but before $form->isValid().

Use cases:

• Perform complex cross-field validation not easily handled by Constraints
• Return a custom Response (e.g., for AJAX/Vue integrations)
• Modify entity or form data dynamically before validation runs

Usage example:

use EasyCorp\Bundle\EasyAdminBundle\Event\BeforeFormValidateEvent;

#[AsEventListener]
public function onBeforeFormValidate(BeforeFormValidateEvent $event): void
{
    $entity = $event->getEntityInstance();
    $form = $event->getForm();
    
    // Custom validation logic
    if ($someCondition) {
        $event->setResponse(new JsonResponse(['error' => 'Custom error']));
    }
}

Changes

• New BeforeFormValidateEvent class extending AbstractLifecycleEvent with StoppableEventTrait
• Event dispatched in AbstractCrudController::new() and edit() actions

Test plan

• Manual testing with an event listener
• Verify existing new/edit flows still work

Fix #7509

[pkly]

Would you consider adding events for after the form is submitted and validated while at it? #7254

There's a small use case for it where we add custom fields, but there's no easy way of reading it back.
Also - maybe consider passing AdminContext along to the event?

[Amoifr]

Thanks @pkly, both suggestions are spot on! Passing AdminContext and having a post-validation event would definitely make the feature more complete.

@javiereguiluz — what's your preference here? Should I keep this PR focused on BeforeFormValidateEvent and handle the rest in a follow-up (new issue/PR), or would you rather I bundle everything together?