Unescaped data going to XMP metadata makes output documents unparseable

[epuronta]

When generating XMP metadata in drafthorse/pdf.py:158, the data is embedded in the XML template string unescaped. Now, if any non-XML-safe data comes in, the output XMP is invalid and will make the output PDF unparseable as Factur-X.

This will produce something like this when reading the file back:
pypdf.errors.PdfReadError: XML in XmpInformation was invalid: not well-formed (invalid token)

Or when the file is handled with Mustang:

WARNING: Problems with parsing metadata. XML parsing failure
org.verapdf.xmp.XMPException: XML parsing failure
...
Caused by: org.xml.sax.SAXParseException; lineNumber: 17; columnNumber: 28; The entity name must immediately follow the '&' in the entity reference.

There are two easy cases to make this occur in practice with metadata automatically extracted from Factur-X payload in drafthorse/pdf.py:294

• Selling company name in ApplicableHeaderTradeAgreement/SellerTradePartyName/Name. Generating invoices for Michael & Sonwill fail
• Invoice number in ExchangedDocument/ID. Less likely, but still possible. Fails with an invoice number like A&A-1

All metadata going to XMP generation should be escaped.

I'll be creating a PR to fix it.