Add "Suppress Discovery" field to the SourceRecord data structure

TODO:
Discuss details how this field can be changed.

This will be a flag in non-MARC data about the MARC record. It can be acted on from the MARCcat side, but also maybe from the Inventory side. This will allow a record to be suppressed from output for OAI-PMH in the future.

Define a separate container for non-MARC data or metadata related to MARC record. "suppress from discovery flag" is the first field to add there.