[VARIANT] Add support for DataType::Struct for cast_to_variant

[carpecodeum]

Which issue does this PR close?

• Closes [Variant]: Implement DataType::Struct support for cast_to_variant kernel #8061

Rationale for this change

Add support for DataType::Struct for cast_to_variant

What changes are included in this PR?

Adds support for casting and adds tests as well

Are there any user-facing changes?

yes casting to variant is a user facing issue

Props to @mprammer!!

[alamb]

Thanks @carpecodeum -- I think this implementation may have some non trivial performance issues, but we can also address that as a follow on PR

Just let me know how you want to proceed:

1. Merge this PR and file a follow on ticket (my preference)
2. Try and improve the performance in this PR

[alamb]

I think as written this will cast the entire input array for each row, and then only read a single row each time, which will likely perform pretty poorly

One way to avoid this might be to slice the field array and only convert the slice (e.g.

let field_array = field_array.slice(i, 1);
let field_variant_array = cast_to_variant(field_array)?;
let field_variant = field_variant_array.value(0); // index zero

Another way that might be even faster could be to recursively cast each field once at the start of the function in a pre-traversal order, and then traverse the input fields the same way (so you always had access to the current field as a variant)

[carpecodeum]

Thanks @carpecodeum -- I think this implementation may have some non trivial performance issues, but we can also address that as a follow on PR

Just let me know how you want to proceed:

1. Merge this PR and file a follow on ticket (my preference)
2. Try and improve the performance in this PR

Thank you for reviewing, I will make the changes here

[alamb]

Unfortunately this branch seems to have conflicts again

[alamb]

🚀

[alamb]

Thanks @carpecodeum