Add CLI for Factur-X / ZUGFeRD

[liZe]

The new API is:

weasyprint invoice.html invoice.pdf --attachment=factur-x.xml --attachment-relationship=Data --xmp-metadata=rdf.xml --pdf-variant=pdf/a-3a

The Python API has also been simplified:

from weasyprint import Attachment, HTML

document = HTML("invoice.html").render()

factur_x_xml = Path("factur-x.xml").read_text()
attachment = Attachment(string=factur_x_xml, name="factur-x.xml", relationship="Data")
document.metadata.attachments = [attachment]

xmp_metadata = Path("rdf.xml").read_text().encode()
document.metadata.xmp_metadata = [xmp_metadata]

document.write_pdf("invoice.pdf", pdf_variant="pdf/a-3b")

Fix #2583.