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Add an extid_version field to ExtIDs

Description

Add an extid_version field to ExtIDs

This allows distinguishing multiple potential versions of the mapping
between external objects and their counterparts archived in Software
Heritage, for instance when a loader has a backwards-incompatible change
that should result in objects being loaded again.

The field defaults to zero, in which case it's backwards-compatible with
the previous implementation in terms of identifier computation.