feat(libreoffice): add generateTaggedPdf form field

Tags the PDF without declaring PDF/UA conformance. Today tagging is only
available through pdfua, which also writes pdfuaid:part=1 into the XMP
metadata. Mirrors the Chromium routes, where generateTaggedPdf and pdfua
are separate fields. Defaults to false, so existing requests are
unchanged.

Closes #1676

.bruno/LibreOffice/Convert to PDF.bru

@@ -31,6 +31,7 @@ body:multipart-form {
   ~skipEmptyPages: false
   ~addOriginalDocumentAsStream: false
   ~singlePageSheets: false
+  ~generateTaggedPdf: false
   ~initialView: 0
   ~initialPage: 1
   ~magnification: 0