Added content_metadata logic to the storage References T733
Summary:
- added tables content_metadata, origin_metadata and origin_metadata_history to db schema
- added constraints for content_metadata in indexes.sql
- added functions in swh-func.sql, db.py, storage.py
(T733 items 1-4)
TODO:
- write constraints for origin_metadata and origin_metadata_history
- add tests
- add tool data in swh-data.sql
- add json schema in json/