[SL-1875] Add a test SqlClient implementation against DuckDB to provide full support for rapid test iteration on MetricFlow itself

[tlento]

We have had a handful of scenarios where dbt-duckdb didn't work with our test configurations across dbt-core version boundaries. It might be nice to have a pure DuckDB SqlClient implementation we can use for testing.

That said, with dbt-core 1.8, adapter packages should be more robustly compatible with subsequent dbt-core upgrades as a general rule, so duckdb might benefit from this change enough that nothing further is needed on our end.

_SL-1875