Support for converting DuckDB STRUCT type to Postgres composite ROW type

[JelteF]

Description

DuckDB has anonymous STRUCT types, with named fields. Postgres doesn't have an equivalent type. The best we can probably do is use the ROW type. This is anonymous compound type, but the fields are not named.

Once supported we can un-exclude the STRUCT types from this test:

pg_duckdb/test/regression/expected/test_all_types.out

Lines 15 to 16 in 02ebeae

	struct,
	struct_of_arrays,

[destrex271]

@JelteF picking this up