Feature: Add Support for preserving array keys as Child <item> Elements in XML Encoder

This PR introduces a new option to the Symfony Serializer's XmlEncoder that allows indexed arrays to be encoded as <item> elements under a single parent element, rather than repeating the parent element for each array item.

Motivation

Currently, encoding an array like:

['person' => [
    ['firstname' => 'Benjamin', 'lastname' => 'Alexandre'],
    ['firstname' => 'Damien', 'lastname' => 'Clay'],
]]

produces the following XML:

<response>
    <person>
        <firstname>Benjamin</firstname>
        <lastname>Alexandre</lastname>
    </person>
    <person>
        <firstname>Damien</firstname>
        <lastname>Clay</lastname>
    </person>
</response>

This structure is not ideal when a clear container-child relationship is required, such as when interoperating with systems that expect such XML structures.

New Behavior

With the new XmlEncoder::PRESERVE_NUMERIC_KEYS option enabled, the same data now encodes to:

<response>
    <person>
        <item key="0">
            <firstname>Benjamin</firstname>
            <lastname>Alexandre</lastname>
        </item>
        <item key="1">
            <firstname>Damien</firstname>
            <lastname>Clay</lastname>
        </item>
    </person>
</response>

Usage

$xml = $encoder->encode($data, 'xml', [
    XmlEncoder::PRESERVE_NUMERIC_KEYS => true,
]);