Add a 'unique_key' method on model objects
that returns a value suitable for unicity constraints.
Motivation:
- this is somewhat more of a model concern than a journal/kafka concern IMO
- this is one step toward adding support for non-model objects in KafkaJournalWriter
Implementation of the unique_key methods comes from
swh.journal.serializers.object_key
.
Migrated from D4078 (view on Phabricator)