Thoth metadata export to support ISBN registration

[tosteiner]

It would be great if Thoth would be able to generate an ONIX export that covers the variety of fields needed for registering ISBNs for a given book.

Relates to @amandasramalho's explorations of the Latin American ISBN registration landscape, see e.g.

• a mapping of Brazilian ISBN fields to Thoth
• as well as similar files for Argentina, plus the CERLALC system which is used by 17 countries in LA

available on our Nextcloud, at https://cloud.obpcloud.org/f/17394

Also see the ISBN Manual provided by the international ISBN agency, partic. chapter 8 "Register of ISBNs and accompanying metadata", available in a variety of languages at https://www.isbn-international.org/content/isbn-users-manual/29

There also is a note indicating that "The International ISBN Agency will supply an XML schema on request"

[ja573]

To check if #441 satisfies the requirements after it's implemented