feature #36186 [FrameworkBundle] Dump kernel extension configuration (guillbdx)

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

Discussion
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[FrameworkBundle] Dump kernel extension configuration

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no
| New feature?  | yes
| Deprecations? | no
| Tickets       | #34756
| License       | MIT

If the kernel is a container extension and defines a configuration, the `config:dump-reference` will now be able to dump it.

Commits
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2ccafb1eb3 [FrameworkBundle] Dump kernel extension configuration

Author: nicolas-grekas

Compiler/ValidateEnvPlaceholdersPass.php

@@ -12,6 +12,7 @@
 namespace Symfony\Component\DependencyInjection\Compiler;
 
 use Symfony\Component\Config\Definition\BaseNode;
+use Symfony\Component\Config\Definition\ConfigurationInterface;
 use Symfony\Component\Config\Definition\Processor;
 use Symfony\Component\DependencyInjection\ContainerBuilder;
 use Symfony\Component\DependencyInjection\Extension\ConfigurationExtensionInterface;
@@ -68,14 +69,18 @@ public function process(ContainerBuilder $container)
             $processor = new Processor();
 
             foreach ($extensions as $name => $extension) {
-                if (!$extension instanceof ConfigurationExtensionInterface || !$config = array_filter($container->getExtensionConfig($name))) {
+                if (!($extension instanceof ConfigurationExtensionInterface || $extension instanceof ConfigurationInterface)
+                    || !$config = array_filter($container->getExtensionConfig($name))
+                ) {
                     // this extension has no semantic configuration or was not called
                     continue;
                 }
 
                 $config = $resolvingBag->resolveValue($config);
 
-                if (null === $configuration = $extension->getConfiguration($config, $container)) {
+                if ($extension instanceof ConfigurationInterface) {
+                    $configuration = $extension;
+                } elseif (null === $configuration = $extension->getConfiguration($config, $container)) {
                     continue;
                 }