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.

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Migrated from D4078 (view on Phabricator)