Clean internal implementation of WindowUDF to use WindowUDFImpl (rather than the function pointers) #8734

Is your feature request related to a problem or challenge?

Similiarly to #8712 (see the rationale there)

@guojidan made a trait for creating WindowUDFs (thank you!) -- #8719 but the existing WindowUDF struct still uses function pointers for implementation.

Describe the solution you'd like

I would like to apply the same basic pattern as in #8713 to WindowUDF so that the implementation of WindowUDF is an inner: Arc<WindowUDFImpl> rather than

Describe alternatives you've considered

No response

Additional context

No response