Improve python partitioner callback interface

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 https://github.com/FEniCS/dolfinx/pull/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.