Implement database schema backup

[FedericoCeratto]

Improve https://github.com/ooni/backend/blob/master/analysis/ooni_db_backup.py backing up "CREATE TABLE ..." for all tables.

[hellais]

We are moving towards using postgresql for everything that cannot be rebuilt from source, see: #807. When we do that this will no longer be needed since postgresql is a managed instance that's already backed up automatically.

For clickhouse we should ensure that the DB schema creation is handled as part of the relevant service component in the code.