[C++][Python] Allow pyarrow.compute.mode to include null count

### Describe the enhancement requested

There is a `skip_nulls` argument to dictate whether nulls should make the result null or be skipped, but it would be potentially be useful for `mode` to return whether null was a mode of the dataset

### Component(s)

Python

Start with the Python component and the pyarrow.compute.mode entry point, then trace how its skip_nulls argument currently affects the result. Check the existing mode behavior and tests before deciding where the null count belongs. Done means mode can report whether null is a dataset mode while retaining the documented skip_nulls behavior.

Written by the indexing model from the issue text.