feature #19326 [Serializer][FrameworkBundle] Add a YAML encoder (dunglas)

This PR was squashed before being merged into the 3.2-dev branch (closes #19326).

Discussion
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[Serializer][FrameworkBundle] Add a YAML encoder

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | todo

Add YAML support to the Serializer.

Commits
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9366a7d [Serializer][FrameworkBundle] Add a YAML encoder

Author: fabpot

DependencyInjection/FrameworkExtension.php

@@ -26,11 +26,13 @@
 use Symfony\Component\HttpKernel\DependencyInjection\Extension;
 use Symfony\Component\Config\FileLocator;
 use Symfony\Component\PropertyAccess\PropertyAccessor;
+use Symfony\Component\Serializer\Encoder\YamlEncoder;
 use Symfony\Component\Serializer\Mapping\Factory\CacheClassMetadataFactory;
 use Symfony\Component\Serializer\Normalizer\DataUriNormalizer;
 use Symfony\Component\Serializer\Normalizer\DateTimeNormalizer;
 use Symfony\Component\Serializer\Normalizer\JsonSerializableNormalizer;
 use Symfony\Component\Workflow;
+use Symfony\Component\Yaml\Yaml;
 
 /**
  * FrameworkExtension.
@@ -1037,6 +1039,12 @@ private function registerSerializerConfiguration(array $config, ContainerBuilder
             $definition->addTag('serializer.normalizer', array('priority' => -900));
         }
 
+        if (class_exists(YamlEncoder::class) && defined('Symfony\Component\Yaml\Yaml::DUMP_OBJECT')) {
+            $definition = $container->register('serializer.encoder.yaml', YamlEncoder::class);
+            $definition->setPublic(false);
+            $definition->addTag('serializer.encoder');
+        }
+
         $loader->load('serializer.xml');
         $chainLoader = $container->getDefinition('serializer.mapping.chain_loader');