Support `UniqueColumn::try_update`

[Centril]

Since there's ::try_insert, we'd also like a try_update to fit with the new update ABI.

[jdetter]

Another request for this was mentioned in the discord https://discord.com/channels/1037340874172014652/1037343189339742298/1361443218499174545

[tamaro-skaljic]

This can be closed as per https://spacetimedb.com/features/requests/11