feat: support Spark-compatible json_tuple function

[CuteChuanChuan]

Which issue does this PR close?

• Part of [EPIC] Complete datafusion-spark Spark Compatible Functions #15914
• Related comet issue: [Feature] Support Spark expression: json_tuple datafusion-comet#3160

Rationale for this change

• Apache Spark's json_tuple extracts top-level fields from a JSON string.
• This function is used in Spark SQL and needed for DataFusion-Comet compatibility.
• Reference: https://spark.apache.org/docs/latest/api/sql/index.html#json_tuple

What changes are included in this PR?

• Add Spark-compatible json_tuple function in datafusion-spark crate
• Function signature: json_tuple(json_string, key1, key2, ...) -> Struct<c0: Utf8, c1: Utf8, ...>
  • json_string: The JSON string to extract fields from
  • key1, key2, ...: Top-level field names to extract
  • Returns a Struct because DataFusion ScalarUDFs return one value per row; caller (Comet) destructures the fields

Examples

SELECT json_tuple('{"f1":"value1","f2":"value2","f3":3}', 'f1', 'f2', 'f3');
-- {c0: value1, c1: value2, c2: 3}

SELECT json_tuple('{"f1":"value1"}', 'f1', 'f2');
-- {c0: value1, c1: NULL}

SELECT json_tuple(NULL, 'f1');
-- NULL

Are these changes tested?

• Unit tests: return_field_from_args shape validation and too-few-args error
• sqllogictest: test_files/spark/json/json_tuple.slt, test cases derived from Spark JsonExpressionsSuite

Are there any user-facing changes?

Yes.