feat: `FunctionRegistry` should expose methods to return names of udaf and udwf

[milenkovicm]

Is your feature request related to a problem or challenge?

At the moment FunctionRegistry exposes funciton udfs which returns names of scalar user defined functions, but there is no similar method for aggregation or window functions. We should provide functions to expose those methods as well

Describe the solution you'd like

provide FunctionRegistry::udafs and FunctionRegistry::udws function which return names of aggregate and window user defined function. Also it will align with. This will also align interfaces with ContextProvider as mentioned in discord by @Omega359

Describe alternatives you've considered

No response

Additional context

This would provide workaround for ballista to support datafusion 50 until we release backward incompatible change described in #17596