Introduce `reverse_expr` for UDAF

[jayzhan211]

Which issue does this PR close?

Closes #. Part of #10091

Rationale for this change

reverse_expr has a different meaning between returning clone and None. Together with reversing, there are three cases. I think the proposed Enum makes the user understand what they can do in reverse_expr.

What changes are included in this PR?

Are these changes tested?

I added a simple UDAF for testing. I want to avoid adding too many other things just for test only because most of them can be covered after moving UDAF from built-in.

Are there any user-facing changes?

[alamb]

Thanks @jayzhan211 -- I'll check this out tomorrow morning