feature #31220 [TwigBridge] bootstrap4 file_widget: allow setting lab…

…el attributes declared in label_attr (AngelFQC)

This PR was submitted for the 4.2 branch but it was merged into the 4.3-dev branch instead (closes #31220).

Discussion
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[TwigBridge] bootstrap4 file_widget: allow setting label attributes declared in label_attr

| Q             | A
| ------------- | ---
| Branch?       | 4.2 <!-- see below -->
| Bug fix?      | yes
| New feature?  | no <!-- don't forget to update src/**/CHANGELOG.md files -->
| BC breaks?    | no     <!-- see https://symfony.com/bc -->
| Deprecations? | no <!-- don't forget to update UPGRADE-*.md and src/**/CHANGELOG.md files -->
| Tests pass?   | yes    <!-- please add some, will be required by reviewers -->
| Fixed tickets | ...   <!-- #-prefixed issue number(s), if any -->
| License       | MIT
| Doc PR        | ... <!-- required for new features -->

> Bootstrap provides a way to translate the “Browse” text in HTML with the data-browse attribute which can be added to the custom input label.

https://getbootstrap.com/docs/4.3/components/forms/#translating-or-customizing-the-strings-with-html

But currently the attributes declared in label_attr aren't being added.

Previously I sent this same change to the master branch (PR #31102), but I think I should have sent it to branch 4.2

Commits
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9c76b29 [TwigBridge] bootstrap4 file_widget: allow setting label attributes declared in label_attr

src/Symfony/Bridge/Twig/Resources/views/Form/bootstrap_4_layout.html.twig

@@ -122,7 +122,8 @@
     <{{ element|default('div') }} class="custom-file">
         {%- set type = type|default('file') -%}
         {{- block('form_widget_simple') -}}
-        <label for="{{ form.vars.id }}" class="custom-file-label">
+        {%- set label_attr = label_attr|merge({ class: (label_attr.class|default('') ~ ' custom-file-label')|trim }) -%}
+        <label for="{{ form.vars.id }}" {% with { attr: label_attr } %}{{ block('attributes') }}{% endwith %}>
             {%- if attr.placeholder is defined -%}
                 {{- translation_domain is same as(false) ? attr.placeholder : attr.placeholder|trans({}, translation_domain) -}}
             {%- endif -%}