feat: implement `n_unique` for DuckDB #5687

Job	Run time
pytest-38 (3.8, windows-latest)	1m 12s
pytest-full-coverage (3.11, ubuntu-latest)	4m 17s
pytest-38 (3.8, ubuntu-latest)	46s
pytest-full-coverage (3.13, ubuntu-latest)	3m 52s
pytest-windows (3.10, windows-latest)	4m 34s
pytest-windows (3.12, windows-latest)	5m 11s
	19m 52s

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