create origin_metadata table

origin_metadata will keep metadata about origins from different sources (lister, loader, sword-deposit, external-catalogs) at a given time.

NOTE: a translation to the CodeMeta vocabulary will be implemented on the fly with the "swh-metadata-translator"

references $166 references swh/devel/swh-deposit#717 (closed)

• storage.py (as the entry point of storage) add, get
• db.py (as the entry point to db)
• swh-func.sql (the actual sql procedure) -> not needed
• add the table itself to swh-schema.sql and constraints to swh-indexes.sql
• swh.storage.api.{server,client}
• add json schema into sql/json
• tests to test add and get origin_metadata

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Migrated from T737 (view on Phabricator)