[DF] Add handling for overloaded UDFs

[charlesbluca]

This PR changes the implementation of DaskFunctions to support overloaded UDF definitions:

• the return_type attribute has been replaced with return_types, a HashMap<Vec<PyDataType>, PyDataType>, mapping the potential input types of a UDF to its return type
• this mapping is updated whenever an overloaded UDF definition is introduced
• the mapping is used as part of the ScalarUDF's ReturnTypeFunction to assert which return type should be used for the given UDF call

Additional things that need to be done:

• [ ] clean up the implementation to cut down on unnecessary clones
• [ ] add handling to drop DaskFunctions when replace=True for register_function