add content_metadata_add/missing/get into:

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