Add `FunctionRegistry::register_udaf` and `FunctionRegistry::register_udwf`

[alamb]

Is your feature request related to a problem or challenge?

Part of #8045

Eventually I think we should remove the distinction between built in and user defined aggregate functions #8708 and window functions #8709, as we are doing for scalar functions

In the interim it is also nice to have the APIs for scalar functions be symmetric with aggregate functions

Describe the solution you'd like

Add functions similarly to FunctionRegistry::register_udf

Describe alternatives you've considered

No response

Additional context

No response