feat: add object_agg transformation

[ilkinulas]

Adds support for Snowflake's OBJECT_AGG aggregate function, which builds a JSON object from key-value pairs.

Example

Snowflake query:

SELECT id, OBJECT_AGG(key_col, value_col) AS obj
FROM test_table GROUP BY id ORDER BY id;

Duckdb equivalent:

SELECT id, TO_JSON(MAP(
    LIST(key_col) FILTER (WHERE NOT key_col IS NULL AND NOT value_col IS NULL),
    LIST(value_col) FILTER (WHERE NOT key_col IS NULL AND NOT value_col IS NULL)
)) AS obj
FROM test_table GROUP BY id ORDER BY id;

• Uses FILTER to skip NULL values, matching Snowflake's default behavior
• Wraps values in TO_JSON to preserve their original types
• Follows the same pattern as the existing array_agg transformation