Add a `AggregateUDFImpl::simplfy()` API

[alamb]

Is your feature request related to a problem or challenge?

For the same reasons as described for ScalarUDFImpl on #9289 we should allow user defined aggregates to provide simplification rules

Describe the solution you'd like

Add AggregateUDFImpl::simplify, following the model @jayzhan211 made in
#9304

Describe alternatives you've considered

No response

Additional context

No response

[yyy1000]

I want to try it, maybe I can learn from #9304

[alamb]

Thanks @yyy1000 -- I think the first thing to do would be to pick an example to deonstrate how it works

Perhaps you could implement a case to simplify COUNT(NULL) to 0 or something?

[yyy1000]

Ok! I will try it