Is your feature request related to a problem or challenge?

We have recently issues show that it is time to support Union

#10161 #10139 ScalarValue::iter_to_array
#10180 comparison_coercion

I think the first step is able to create Union in slt file.

Describe the solution you'd like

Follow DuckDB Union or others well-known system.

https://duckdb.org/docs/sql/data_types/union.html

First step might be able to create table with union type

query error DataFusion error: Error during planning: Inconsistent data type across values list at row 1 column 0\. Was Int64 but found Utf8
create table t1 (u union(num interger, str varchar)) as values
  (1),
  ('two');

Describe alternatives you've considered

No response

Additional context

1. Built Union on top of Struct
2. Impl with ScalarUDFImpl