[C++] [Dataset] Support max file size option in write dataset

The existence FileSystemDatasetWriteOptions::basename_template would seem to imply that the dataset writer may write multiple files for a given partition.  However, the current implementation will always create one file per directory.

I'm not sure what the desired behavior is here but the two obvious choices are:

 * Get rid of FileSystemDatasetWriteOptions::basename_template (or at least the {i} parameter)

 * Add an option to limit how many rows/bytes are put in a single file

ARROW-12358 is probably worth mentioning as whatever strategy is come up with here should probably be compatible with supporting append mode in the future.