Support arrays with a fixed length

[JelteF]

Description

DuckDB allows arrays to have a fixed length. But returning those from a query currently throws an error that we don't support the type.

select * from duckdb.query($$ select [1, 2]::int[2] $$);
WARNING:  01000: (PGDuckDB/GetPostgresDuckDBType) Could not convert DuckDB type: INTEGER[2] to Postgres type
LOCATION:  GetPostgresDuckDBType, pgduckdb_types.cpp:1098
ERROR:  XX000: (PGDuckDB/CreatePlan) Cache lookup failed for type 0
LOCATION:  CreatePlan, pgduckdb_planner.cpp:76

It should be pretty simple to support these by simply converting them to a postgres array type (without a specified length). i.e. convert INTEGER[2] to INTEGER[] automatically.

Once we do this we should be able to un-exclude a bunch of these lines:

pg_duckdb/test/regression/expected/test_all_types.out

Lines 21 to 27 in 02ebeae

	fixed_varchar_array,
	fixed_nested_int_array,
	fixed_nested_varchar_array,
	fixed_struct_array,
	struct_of_fixed_array,
	fixed_array_of_int_list,
	list_of_fixed_int_array,