Introduce SeekRowGroup API

[zolstein]

Add a new method, SeekRowGroup, to the ParquetReader that logically moves the reader to the start of the indexed row group, such that the next read will start by returning the first row of that row group.

This method does no I/O on the underlying file, such that clients can skip over row groups that contain records that they don't need to read without needing to do work proportional to the size of the file.