Add API to find bound-with item(s) by holdings record ID

Under a holdings record in the Instance details view, the Inventory UI currently displays Items directly associated with a holdings record by foreign key (in a many to one relation that is).

With the introduction of bound-withs an Item can reversely be a gathering of many holdings records (and thus many titles). The Inventory UI needs display these bound-with Items too below each of the holdings records that are part of the bound-with even if the Item is not directly related to the holdings record by a foreign key.

For this, a new back-end API is required, that will find and retrieve an Item, that represents a bound-with, by the holdings record ID of one of the parts.

For example:
(the pieces that are involved in bound-withs are marked with a "(B)", see also MODINV-388)

First quarter, 2020 issue of magazine, the item is the select bound-with item directly related to the holdings record for that issue (pulled in by foreign key). The item's barcode is "2020q1c1":

Second quarter, same year, one copy is bound together with other quarterly issues within the above item. For that copy, the item under the holdings record is the same item as above: "2020q1c1":