…to ease configuration tree manipulation (jschaedl)

This PR was squashed before being merged into the 4.4 branch (closes #31287).

Discussion
----------

[Config] Introduce find method in ArrayNodeDefinition to ease configuration tree manipulation

| Q             | A
| ------------- | ---
| Branch?       | 4.4
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| 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 | #27534   <!-- #-prefixed issue number(s), if any -->
| License       | MIT
| Doc PR        | tbd.

### Description

This PR introduces a new `find(string $nodePath)`method in the `ArrayNodeDefinition` class, which helps you finding the right node to prepend configuration to ease configuration tree manipulation.

### How to use it
```php
class Configuration implements ConfigurationInterface
{
    public function getConfigTreeBuilder()
    {
        ...

        $rootNode
            ->children()
                ->arrayNode('social_media_channels')
                    ->children()
                        ->booleanNode('enable')->end()
                        ->arrayNode('twitter')->end()
                        ->arrayNode('facebook')->end()
                        ->arrayNode('instagram')->end()
                    ->end()
                ->end()
            ->end()
        ;

        $this->changeSocialMediaChannelConfiguration($rootNode->find('social_media_channels.enable'));
        $this->addTwitterConfiguration($rootNode->find('social_media_channels.twitter'));
        $this->addFacebookConfiguration($rootNode->find('social_media_channels.facebook'));
        $this->addInstagramConfiguration($rootNode->find('social_media_channels.instagram'));

        return $treeBuilder;
    }

    private function changeSocialMediaChannelConfiguration(NodeDefinition $node)
    {
        $node
            ->defaultTrue()
        ;
    }

    private function addTwitterConfiguration(NodeDefinition $node)
    {
        $node
            ->children()
                ->integerNode('client_id')->end()
                ->scalarNode('client_secret')->end()
            ->end()
        ;
    }

    private function addFacebookConfiguration(NodeDefinition $node)
    {
        $node
            ->children()
                ->integerNode('client_id')->end()
                ->scalarNode('client_secret')->end()
            ->end()
        ;
    }

    private function addInstagramConfiguration(NodeDefinition $node)
    {
        $node
            ->children()
                ->integerNode('client_id')->end()
                ->scalarNode('client_secret')->end()
            ->end()
        ;
    }
}

```

Commits
-------

e3b248a [Config] Introduce find method in ArrayNodeDefinition to ease configuration tree manipulation