Integrate ephemeral database(s) with CI for read/write testing

[alexander-beedie]

Description

@stinodego: As promised ;)

Currently read_database and write_database unit tests rely on SQLite when running on CI, which is insufficient to provide quality testing due to the numerous differences between that and something like PostgreSQL, MSSQL, Redshift, Snowflake, etc. While extending these functions I've had to rely on validating new functionality against a containerised PostgreSQL (and a few other DBs) running off a NAS at home, or using Polars at work where we have data in (amongst others) Redshift, Databricks, and BigQuery stores.

We should enhance CI (and the associated unit tests) to, at a minimum, support an ephemeral PostgreSQL test target as well. If there are other databases that support simple containerisation (or some equivalent facility) then we could think about adding more than PostgreSQL later 🤔