[GSI] Document a new stat available for knowing last scan time of an index

Linked improvement: MB-29735

We have provided option to find unused indexes in GSI through a stat. Exposed a new stat last_known_scan_time which is time of last known scan time as an int64 Unit timestamp.
Flat file persistence of selected stats was introduced for this RMS improvement.
Persistence interval of the stat 15 mins. This stat also exposed on public stats REST endpoint