add content_metadata_add/missing/get into:
- [x] storage.py (as the entry point of storage)
- [x] db.py (as the entry point to db)
- [x] swh-func.sql (the actual sql procedure)
- [x] add the table itself to swh-schema.sql and constraints to swh-indexes.sql
- [x] the data about the indexer tool into swh-data.sql
- [x] swh.storage.api.{server,client}
- [ ] add json schema into sql/json
- [x] tests to test add and get content_metadata
- [x] add db_to_metadata in converters