Add additional backends

Since we already have one for MariaDB/MySQL, adding one for PostgreSQL should not be too hard.

These also look interesting:
- https://github.com/canonical/dqlite
- https://github.com/rqlite/rqlite
- https://github.com/cockroachdb/cockroach
- https://github.com/yugabyte/yugabyte-db
- https://www.ferretdb.com/
- https://github.com/hse-project/hse