Create instanceRelationship objects for parent-child instance relationships as part of items transformer

When performing an item transform task we need to (optionally?) create parent-child instanceRelationship objects (instead of boundwithItems) under the following circumstances:

1. An item is associated with more than one Bib ID
2. One of those Bib IDs is associated with more than one item

In this case, a holdings relationship should not be created for the matching Bib ID. Instead, an "instanceRelationship" object should be created between that Bib's instance and the other Bib ID instances associated with the item record.

Questions:

- Should this be the standard handling or optional?
- Is this a generally applicable use-case, or Sierra/Millenium specific?
- Is the algorithm for identifying the parent instance sound?
- …???

Start by locating the items transformer and reviewing how it currently creates boundwithItems and holdings relationships. Resolve whether instanceRelationship creation is optional, whether the use case is Sierra/Millennium-specific, and how the parent instance is identified before defining the expected transformed output.

Written by the indexing model from the issue text.