[C++][Dataset] Allow setting FragmentReadahead to 0 in ScannerBuilder

I have an application where I need to set fragment readahead to 0. But, looks like for some reason the ScannerBuilder does not allow setting the fragment readahead to 0 [1]. It would be very helpful to know why it is that way and if a PR lifting that restriction would be accepted because a docstring mentions that users can set fragment readahead to 0 if they want [2].

[1]https://github.com/apache/arrow/blob/master/cpp/src/arrow/dataset/scanner.cc#L864
[2]https://github.com/apache/arrow/blob/998a2a1668ea57a49d85fbb38f7f0e7eb94c29db/cpp/src/arrow/dataset/scanner.h#L93