feature #31375 [Form] Add label_html attribute (przemyslaw-bogusz)

This PR was squashed before being merged into the 5.1-dev branch.

Discussion
----------

[Form] Add label_html attribute

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT
| Doc PR        | symfony/symfony-docs#13316

I propose to add a new attribute to `BaseType` class so it is easy to include html tags in labels for, both, buttons and other elements that inherit from `FormType` class. This gives you an ability to add, e.g. a glyphicon to a button, or a link to a checkbox, simply inside the `FormBuilder`, which means you can just do
```twig
{{ form(form) }}
```
inside a template.

**Sidenotes**
1. I have started working on this two days ago and it the meantime @alexander-schranz made a similar proposition in #31358. If necessary, I can close my PR and @alexander-schranz can include my suggestions inside his PR.
1. I have just read in #29861 that @mpiot wanted to include this idea in his PR. With respect to @xabbuh's comments from that PR, I hope that my PR will be at least a good place to discuss, if the proposed feature is a good solution.

Commits
-------

239fe04ff9 [Form] Add label_html attribute

Author: nicolas-grekas

Extension/Core/Type/BaseType.php

@@ -97,6 +97,7 @@ public function buildView(FormView $view, FormInterface $form, array $options)
             'disabled' => $form->isDisabled(),
             'label' => $options['label'],
             'label_format' => $labelFormat,
+            'label_html' => $options['label_html'],
             'multipart' => false,
             'attr' => $options['attr'],
             'block_prefixes' => $blockPrefixes,
@@ -127,6 +128,7 @@ public function configureOptions(OptionsResolver $resolver)
             'label' => null,
             'label_format' => null,
             'row_attr' => [],
+            'label_html' => false,
             'label_translation_parameters' => [],
             'attr_translation_parameters' => [],
             'attr' => [],
@@ -137,5 +139,6 @@ public function configureOptions(OptionsResolver $resolver)
         $resolver->setAllowedTypes('block_prefix', ['null', 'string']);
         $resolver->setAllowedTypes('attr', 'array');
         $resolver->setAllowedTypes('row_attr', 'array');
+        $resolver->setAllowedTypes('label_html', 'bool');
     }
 }

Tests/Fixtures/Descriptor/resolved_form_type_1.json

@@ -46,6 +46,7 @@
                 "label",
                 "label_attr",
                 "label_format",
+                "label_html",
                 "label_translation_parameters",
                 "mapped",
                 "method",

Tests/Fixtures/Descriptor/resolved_form_type_1.txt

@@ -26,6 +26,7 @@ Symfony\Component\Form\Extension\Core\Type\ChoiceType (Block prefix: "choice")
                                                    label                                                 
                                                    label_attr                                            
                                                    label_format                                          
+                                                   label_html                                            
                                                    label_translation_parameters                          
                                                    mapped                                                
                                                    method                                                

Tests/Fixtures/Descriptor/resolved_form_type_2.json

@@ -26,6 +26,7 @@
             "label",
             "label_attr",
             "label_format",
+            "label_html",
             "label_translation_parameters",
             "mapped",
             "method",

Tests/Fixtures/Descriptor/resolved_form_type_2.txt

@@ -28,6 +28,7 @@ Symfony\Component\Form\Extension\Core\Type\FormType (Block prefix: "form")
   label                         
   label_attr                    
   label_format                  
+  label_html                    
   label_translation_parameters  
   mapped                        
   method