UBLExtensions implementation along with additional namespaces

[hevesi]

Added support for UBLExtensions tag with also adding additional namespaces to the root Invoice element.

UBLExtensions tag was required for croatian e-invoicing.

This tag is optional and the inclusion of adding additional namespaces is required because the extensions can use new namespaces (the croatian FINA fiscalization did use).

Changes

• Added Extension.php class implementing XmlSerializable and XmlDeserializable
• Added addExtension, setExtensions, and getExtensions methods to Invoice.php
• Modified Generator.php to include the ext: namespace when extensions are present and it now accepts additional custom namespaces (e.g., hrextac) via the $additionalNamespaces parameter

Usage Example

$extension = (new \NumNum\UBL\Extension())
    ->setContent([
        'hrextac:HRFISK20Data' => [
            'hrextac:HRTaxTotal' => [ /* ... */ ],
            'hrextac:HRLegalMonetaryTotal' => [ /* ... */ ]
        ]
    ])
    ->setAttributes([
        'xmlns:cct' => 'urn:un:unece:uncefact:data:specification:CoreComponentTypeSchemaModule:2',
        // ...
    ]);

$invoice->addExtension($extension);

$generator->invoice($invoice, 'EUR', [
    'urn:mfin.gov.hr:schema:xsd:HRExtensionAggregateComponents-1' => 'hrextac'
]);