fix(duckdb)!: Fix STRUCT cast generation

[VaggelisD]

Fixes #4365

BigQuery supports inline STRUCT constructors such as the following:

bq> SELECT STRUCT<a STRING, b INTEGER>('str', 1) AS strct;
strct.a   strct.b
'str'        1

#3751 introduced support in SQLGlot by canonicalizing those expressions to a CAST:

>> sqlglot.parse_one("SELECT STRUCT<a STRING, b INTEGER>('str', 1)", read="bigquery").sql("bigquery")
`CAST(STRUCT('str', 1) AS STRUCT<a STRING, b INTEGER>)`

Note thought that transpilation towards DuckDB was still not complete as it requires explicit key-value pairs to construct STRUCTs:

D SELECT CAST(STRUCT_PACK('str', 1) AS STRUCT("a" TEXT, "b" INT));
Binder Error: Need named argument for struct pack, e.g. STRUCT_PACK(a := b)

To solve this issue efficiently, the PR was extended so that DuckDB can understand when a STRUCT was constructed under a STRUCT cast and turn that into CAST(ROW(...) AS STRUCT(...) instead:

D select CAST(ROW('str', 1) AS STRUCT("a" TEXT, "b" INT));
┌────────────────────────────────────────────────────────────┐
│ CAST(main."row"('str', 1) AS STRUCT(a VARCHAR, b INTEGER)) │
│                struct(a varchar, b integer)                │
├────────────────────────────────────────────────────────────┤
│ {'a': str, 'b': 1}                                         │
└────────────────────────────────────────────────────────────┘

However, #4365 shows how this is prone to errors if the ROW field def order does not match with the cast:

D SELECT CAST(ROW(1, 'foo') AS STRUCT("s" TEXT, "i" INT));
Conversion Error: Could not convert string 'foo' to INT32

To mitigate this issue, the #3751 solution can be further limited to generate a ROW only if the STRUCT being casted does not have key - value pairs; This seems enough to (1) preserve existing BQ -> DuckDB transpilation cases and (2) restore DuckDB's roundtrips which are order-agnostic:

D SELECT CAST({'i':1, 's':'foo'} AS STRUCT("s" TEXT, "i" INT));
┌────────────────────────────────────────────────────────────────────────────┐
│ CAST(main.struct_pack(i := 1, s := 'foo') AS STRUCT(s VARCHAR, i INTEGER)) │
│                        struct(s varchar, i integer)                        │
├────────────────────────────────────────────────────────────────────────────┤
│ {'s': foo, 'i': 1}                                                         │
└────────────────────────────────────────────────────────────────────────────┘