Add CONCAT_WS fn to APIv4 and UI support in SearchKit

[colemanw]

Overview

Followup to #21528 this adds a more useful string concat function

[civibot]

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[eileenmcnaughton]

This works - I found it a bit confusing but that might be because it's kinda complicated functionality. Also for some reason my lastpass got a bit too excited - I think some css is in play that affects that? Anyway merging since I think it is a good addition