Add support for ZUGFeRD/XRechnung/Factur-X

[shochdoerfer]

Addition to #318: In the next few years, electronic invoices (ZUGFeRD/XRechnung/Factur-X) will be the standard in the EU. Thus, if we want to migrate away from the current wkhtmltopdf solution, checking our compliance options for electronic invoices makes sense.

In Germany, as of 01.01.2025, companies must be able to receive and process electronic invoices. From 01.01.2026 on, companies with an annual turnover > 800K EUR must send electronic invoices, and as of 01.01.2027, every company must send electronic invoices.

So far, I found the easybill/zugferd-php project helpful in generating the specific XML format that needs to be embedded in a PDF invoice. We would need to figure out how to generate the required PDF format and embed the XML data ourselves. I'm not yet sure if https://gotenberg.dev/ can be of help here.

As an alternative, I found horstoeko/zugferd, which can generate and embed the XML data right into an (existing) PDF document.