Add union_extract kernel

[gstvg]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

Retrieve the value of the given union variant, or NULL if it's not currently selected
This is will be used at apache/datafusion#11081

Describe the solution you'd like

A kernel in arrow_select with the following signature:
fn union_extract(union_array: &UnionArray, field: &str) -> Result<ArrayRef>

Describe alternatives you've considered

Implement this in datafusion, but after comments from @samuelcolvin and @alamb at #6247 (comment) and #6247 (comment), I decided to do here

Additional context

[alamb]

I think this is needed for https://github.com/datafusion-contrib/datafusion-functions-json

I will put it on my review queue

[alamb]

label_issue.py automatically added labels {'arrow'} from #6387