add swh-model CLI front-end to compute persistent identifiers

We want to a generic CLI util that can be distributed to end users to compute SWH persistent identifiers. Intended installation method will be pip install swh-model. The CLI will just be a tiny wrapper around a dedicated entry point for the module. As a starting point we have some scripts under bin/ in swh-model, but they need to be refactored/merged.

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Migrated from T1039 (view on Phabricator)