Support returning OPAC records

That means getting the holdings and items, which is best done by running against mod-graphql and doing an all-in-one query.

This means we want to do different FOLIO WSAPI requests depending on whether the user wants MARC or OPAC records – straight to inventory-storage/instances in the former case, and via GraphQL in the latter. Which means that we don't know what search we should have done until the fetch happens. Not sure what to do about this. We should probably always do the lightweight inventory-storage search, considering it a cheap write-off in the case when we're going to have to do a much more heavyweight instance-and-its-holdings-and-their-items search.

Documentation:
Z39.50 Holdings and Item Data elements - https://docs.google.com/spreadsheets/d/1MnLNwUFnQ-B3gU62B-4CCOnIqXP7fMxdsN5_Pm-cH3I/edit#gid=0
Logfile from Lehigh - metaproxy.log
uChicago - Export file for discovery -