Include sibling title instance details in agreement and agreement line export

Purpose:

On exporting agreements / agreement lines as JSON, the data does not currently include sibling title information, meaning that identifiers related to print versions of titles do not typically export with the agreement / agreement line export

This story is to add  the sibling title information into the export so that the export includes identifiers for both the electronic and print versions of a title (if they are in the local KB)

User story statement(s):

As an erm librarian
I want to export the details of resources in an agreement including identifiers from print versions of titles
so that I can have all relevant identifiers in the export

Scenarios:

1. Scenario:
   • Given
     • Two sibling TIs, one subtype  == electronic, one subtype == print, both with identifiers
     • A PCI for the TI with subtype == electronic
     • The PCI added to an agreement as part of a package in an agreement line
   • When I choose to export the agreement via the /erm/sas/{SAS UUID}/export/current endpoint
   • Then for each title in the export, a "relatedTitles" array will be included with any sibling title instances and their identifiers (relatedTitles are already included in some other endpoint responses including /erm/resource/{title instance UUID}
2. Scenario:
   • Given
     • Two sibling TIs, one subtype  == electronic, one subtype == print, both with identifiers
     • A PCI for the TI with subtype == electronic
     • The PCI added to an agreement directly as an agreement line
   • When I choose to export the agreement via the /erm/sas/{SAS UUID}/export/current endpoint
   • Then for each title in the export, a "relatedTitles" array will be included with any sibling title instances and their identifiers (relatedTitles are already included in some other endpoint responses including /erm/resource/{title instance UUID}
3. Scenario:
   • Given
     • Two sibling TIs, one subtype  == electronic, one subtype == print, both with identifiers
     • A PCI for the TI with subtype == electronic
     • The PCI added to an agreement as part of a package in an agreement line
   • When I choose to export the agreement via the /erm/sas/{SAS UUID}/resources/export/current endpoint
   • Then for each title in the export, a "relatedTitles" array will be included with any sibling title instances and their identifiers (relatedTitles are already included in some other endpoint responses including /erm/resource/{title instance UUID}
4. Scenario:
   • Given
     • Two sibling TIs, one subtype  == electronic, one subtype == print, both with identifiers
     • A PCI for the TI with subtype == electronic
     • The PCI added to an agreement directly as an agreement line
   • When I choose to export the agreement via the /erm/sas/{SAS UUID}/resources/export/current endpoint
   • Then for each title in the export, a "relatedTitles" array will be included with any sibling title instances and their identifiers (relatedTitles are already included in some other endpoint responses including /erm/resource/{title instance UUID}