Dynamically generate MARC records by mapping instances

We will eventually (though not for the MVP) want the Z39.50 server to have the option of generating MARC records on the fly, as opposed to fetching them from SRS. To do this, we will need to take the information in FOLIO inventory instances (and possibly their associated holdings and items), and map it into MARCXML, which the YAZ frontend will then convert into an ISO 2709 record.

One approach to this would be to create and maintain an XSLT stylesheet that maps from an XML representation of FOLIO's JSON records into MARCXML. ZF-8 is about such a stylesheet.

Another approach would be to reuse some part of the inventory-to-MARC mapping work that was done for UXPROD-1397.