Improve python partitioner callback interface

[schnellerhase]

The python callbacks of a graph partitioner currently require a cpp return value type, so either AdjacencyList_int32 or AdjacencyList_int64. With the introduction of a general python wrapper for the adjacency list types in #3509 we should switch the return type of the python callback, that the user needs to implement, to the type oblivious AdjacencyList type.

See TODO at https://github.com/FEniCS/dolfinx/blob/main/python/test/unit/fem/test_dofmap.py#L439

One possible way to achieve this would be with a decorator, as suggested by @jhale.