…ino)

This PR was squashed before being merged into the 5.3-dev branch.

Discussion
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[Form] Add "form_attr" FormType option

| Q             | A
| ------------- | ---
| Branch?       | 5.x
| Bug fix?      |no
| New feature?  | yes
| Deprecations? | no
| Tickets       | N/A
| License       | MIT
| Doc PR        | [#15108](symfony/symfony-docs#15108)

## What is this about

This PR add support for [`form` attribute](https://html.spec.whatwg.org/multipage/form-control-infrastructure.html#attr-fae-form) to Symfony Form ([browser compatibility](https://caniuse.com/form-attribute)).

The `form` attribute allows form elements to override their associated form (which is their nearest ancestor form element by default). This is extremely useful to solve **nested form problem** and allows **form children** to be **rendered outside form tag** while still working as expected.

## New "form_attr" FormType option

#### form_attr
**type**: `bool` or `string` **default**: `false`

If set to `true` on a **root form**, adds [`form` attribute](https://html.spec.whatwg.org/multipage/form-control-infrastructure.html#attr-fae-form) on every children with their root **form id**.
This allows you to render form children outside the form tag and avoid **nested form problem** in some situations while keeping the form working properly.

If set to `true` on a **child**, adds [`form` attribute](https://html.spec.whatwg.org/multipage/form-control-infrastructure.html#attr-fae-form) on it with its **root form id**.
This allows you to render **that child** outside the form tag and avoid **nested form problem** in some situations while keeping the form working properly.

If root form has no `id` (this may happen by create an *unnamed* form), you can set it to a `string` identifier to be used at `FormView` level to link children and root form anyway.

## Usage on Root Form Example

#### Form Type
Enable the feature by setting `form_attr` to `true` on the root form.

```php
use Symfony\Component\Form\Extension\Core\Type;

class ListFilterType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options): void
    {
        $builder
            ->add('search', Type\SearchType::class)
            ->add('orderBy', Type\ChoiceType::class, [
                'choices' => [
                    // ...
                ],
            ])
            ->add('perPage', Type\ChoiceType::class, [
                'choices' => [
                    // ...
                ],
            ])
    }

    public function configureOptions(OptionsResolver $resolver): void
    {
        $resolver->setDefault('form_attr', true); // <--- Set this to true
    }
}

```

#### Twig

The following Twig template **works properly** even if form children are **outside** their form tag ([browser compatibility](https://caniuse.com/form-attribute)).

```twig

<div class="header-filters">
    {{ form_errors(form) }}
    {{ form_row(form.search) }} {# has attribute form="list_filter" #}
    {{ form_row(form.orderBy) }} {# has attribute form="list_filter" #}
</div>

<!-- -->
<!-- Some other HTML content, like a table or even another Symfony form -->
<!-- -->

<div class="footer-filters">
    {{ form_row(form.perPage) }} {# has attribute form="list_filter" #}
</div>
{{ form_start(form) }} {# id="list_filter" #}
{{ form_end(form) }}
```
✅ Every form elements work properly even outside form tag.

## Usage on Form Child Example

Enable the feature by setting `form_attr` to `true` on selected child.

```php
use Symfony\Component\Form\Extension\Core\Type;

class ListFilterType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options): void
    {
        $builder
            ->add('search', Type\SearchType::class)
            ->add('orderBy', Type\ChoiceType::class, [
                'choices' => [
                    // ...
                ],
            ])
            ->add('perPage', Type\ChoiceType::class, [
                'form_attr' => true,  // <--- Set this to true
                'choices' => [
                    // ...
                ],
            ])
    }
}

```

#### Twig

The following Twig template **works properly** even if `form.perPage` is **outside** form tag ([browser compatibility](https://caniuse.com/form-attribute)).

```twig

<div class="header-filters">
    {{ form_start(form) }} {# id="list_filter" #}
        {{ form_errors(form) }}
        {{ form_row(form.search) }}
        {{ form_row(form.orderBy) }}
    {{ form_end(form, {'render_rest': false}) }}
</div>

<!-- -->
<!-- Some other HTML content, like a table or even another Symfony form -->
<!-- -->

<div class="footer-filters">
    {{ form_row(form.perPage) }} {# has attribute form="list_filter" #}
</div>
```
✅ `form.perPage` element work properly even outside form tag.

Commits
-------

5f913ce [Form] Add "form_attr" FormType option