Clean internal implementation of ScalarUDF to use `ScalarUDFImpl` (rather than the function pointers)

[alamb]

Is your feature request related to a problem or challenge?

Follow on to #8578 / #8568

Describe the solution you'd like

Basically now that we have a trait object based way of defining UDFs the internal function pointer implementation is awkward and unecessary. We could use the trait object directly as discussed in https://github.com/apache/arrow-datafusion/pull/8578/files#r1430521000

Describe alternatives you've considered

No response

Additional context

No response

[istvan-fodor]

Hi @alamb, any ideas when this feature will be released? I am working on a project with a lot of custom UDFs and it would be awesome to use the new API.

Thank you!

[alamb]

Hi @alamb, any ideas when this feature will be released? I am working on a project with a lot of custom UDFs and it would be awesome to use the new API.

Thank you!

I believe @andygrove plans to do so soon -- #8863 tracks the release that will have this feature