[C++] Add Compute Kernel for Casting from union to string

[llama90]

Describe the enhancement requested

This is a sub-issue of the issue mentioned below.

• [C++] Remove Scalar legacy cast implementation #35560

This issue is aiming to address #39182.

A pull request (#40237) has been submitted to resolve issue, and additional features that need to be supported have emerged.

From	To	Using Function
sparse_union<string: string=3, number: uint64=42, other_number: uint64=43>	utf8	cast_string
dense_union<string: string=3, number: uint64=42, other_number: uint64=43>	utf8	cast_string

Component(s)

C++

[thisisnic]

This issue has been marked as stale because it has had no activity in the past 365 days. Please remove the stale label or comment below, or this issue will be closed in 14 days. If this improvement is still desired but has no current owner, please add the 'Status: needs champion' label.