[FAETURE] add support for xPath functions

[dkd-kaehm]

There is no way to use the xPath functions in import configs.

Following method and its usages could be adjusted to return and use besides the DOMNodeList the string value:

external_import/Classes/Handler/XmlHandler.php

Lines 359 to 369 in 754e795

	public function selectNodeWithXpath(\DOMXPath $xPathObject, string $xPath, ?\DOMNode $context = null): \DOMNodeList
	{
	$resultNodes = $xPathObject->evaluate($xPath, $context);
	if ($resultNodes->length > 0) {
	return $resultNodes;
	}
	throw new \Exception(
	'No node found with xPath: ' . $xPath,
	1399497464
	);
	}

This will allow composite-identities in configs:

'composite_uid' => [
            'external' => [
                'index_a' => [
                    'xpath' => 'concat(../@parents_element_uid_attr, ":", @child_element_uid_attr)',
                ],
            ],
        ],

Beside of that there is cool feature on core PHP, which could be fine on that extension: https://www.php.net/manual/en/domxpath.registerphpfunctions.php

---

@fsuter @COBWEB
I patched the EXT:external_import minimal(there is a little bit more requiered for full support by EXT:external_import) in my project, would you like to extend the code if I push it and create a pull-request?

[fsuter]

Hi. Thanks for the idea and I would be glad for a PR to get started.