Reporting per column level stats for table

We already have table level and per segment level stats right now.

The next question here is that what's the per column level information/stats we can expose.

One typical question is:
What's the size of this column (total/raw_data/index) at per segment level as well as table level.

Start by locating the existing table-level and per-segment statistics implementations in Apache Pinot and review what information they expose. Clarify the per-column statistics API and the required total, raw-data, and index sizes at both segment and table levels; the issue is done when that scope and behavior are implemented and documented.

Written by the indexing model from the issue text.