Add union_extract function

[gstvg]

Is your feature request related to a problem or challenge?

Retrieve the value of the given union variant, or NULL if it's not currently selected

Describe the solution you'd like

No response

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_extractunion-tag

[gstvg]

take

[tobixdev]

Hi @gstvg!

I have a question regarding this issue. We are currently working on a prototype that would require "ergonomic" handling of unions for filtering etc. I understand that you have implemented a union_extract kernel in arrow-rs. Is there a reason on why the API of using this kernel wasn't merged into DataFusion? Or is there already another way to achieve the semantics of extract_union?

I'd also be happy to give a hand if there's something that I could do.

[gstvg]

Hi @tobixdev

My bad, the arrow kernel ended up being enough for me and I got busy with other work, leaving this for later. I'm already started updating #12116, I should push it soon. I'll ping you there in case you wanna take a look