feat: implement basic struct handling #327

Job	Run time
Test (macos-latest, general, -sql, 3.10, x64)	2m 8s
Test (macos-latest, spark, sql, 3.10, x64)	3m 23s
Test (macos-latest, spark, -sql, 3.10, x64)	5m 9s
Test (macos-latest, gateway-over-duckdb, sql, 3.10, x64)	1m 59s
Test (macos-latest, gateway-over-duckdb, -sql, 3.10, x64)	5m 18s
Test (macos-latest, gateway-over-datafusion, sql, 3.10, x64)	2m 17s
Test (macos-latest, gateway-over-datafusion, -sql, 3.10, x64)	2m 35s
Test (ubuntu-latest, general, -sql, 3.10, x64)	1m 25s
Test (ubuntu-latest, spark, sql, 3.10, x64)	2m 33s
Test (ubuntu-latest, spark, -sql, 3.10, x64)	4m 16s
Test (ubuntu-latest, gateway-over-duckdb, sql, 3.10, x64)	1m 31s
Test (ubuntu-latest, gateway-over-duckdb, -sql, 3.10, x64)	3m 41s
Test (ubuntu-latest, gateway-over-datafusion, sql, 3.10, x64)	1m 24s
Test (ubuntu-latest, gateway-over-datafusion, -sql, 3.10, x64)	2m 27s
	40m 6s

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