[Form][2.3] Fixed Form::submit() and Form::setData() to react to dynamic form modifications

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

ref #3767, #3768, #4548
cc @Burgov

This PR ensures that fields that are added during the submission process of a form are submitted as well. For example:

```php
$builder = $this->createFormBuilder()
    ->add('country', 'choice', ...)
    ->getForm();

$builder->get('country')->addEventListener(FormEvents::POST_SUBMIT, function (FormEvent $event) {
    $form = $event->getForm()->getParent();
    $country = $event->getData();

    $form->add('province', 'choice', /* ... something with $country ... */);
});
```

Currently, the field "province" will not be submitted, because the submission loop uses `foreach`, which ignores changes in the underyling array.

Contrary to #8827 (for 2.2), this PR also allows dynamic modifications during `setData()`:

```php
$builder = $this->createFormBuilder()
    ->add('country', 'choice', ...)
    ->getForm();

$builder->get('country')->addEventListener(FormEvents::POST_SET_DATA, function (FormEvent $event) {
    $form = $event->getForm()->getParent();
    $country = $event->getData();

    $form->add('province', 'choice', /* ... something with $country ... */);
});
```

For 2.2, this fix is not possible, because it would require changing the signature `DataMapperInterface::mapDataToForms($data, array $forms)` to `DataMapperInterface::mapDataToForms($data, array &$forms)`, which would be a BC break. For 2.3, this change is not necessary (instead of an array we now pass an iterator, which is passed by reference anyway).

Additionally, this PR ensures that `setData()` is called on *every* element of a form (except those inheriting their parent data) when `setData()` is called on the root element (i.e., during initialization). Currently, when some of the intermediate nodes (e.g. embedded forms) are submitted with an empty value, `setData()` won't be called on the nodes below (i.e. the fields of the embedded form) until `get*Data()` is called on them. If `getData()` is *not* called before `createView()`, any effects of `*_DATA` event listeners attached to those fields will not be visible in the view.