Add `union_tag` function

[gstvg]

Is your feature request related to a problem or challenge?

Retrieve the name of the currently selected field on a union

Describe the solution you'd like

Return a dictionary encoded string array using the union array type_ids as the dict keys array

Describe alternatives you've considered

No response

Additional context

DuckDB has such function, I propose using the same name
https://duckdb.org/docs/sql/functions/nested.html#union_tagunion

[gstvg]

take