sstable/tieredmeta: add tiering histogram using t-digest #5689
Description
Add StatsHistogram, a quantile sketch backed by t-digest, to track byte distribution by TieringAttribute. The histogram supports CDF and quantile queries to estimate bytes above/below thresholds for tiering decisions.
Add TieringHistogramBlockWriter to encode multiple histograms per sstable or blob file, keyed by (KindAndTier, SpanID). The block will support histogram types for categories necessary for tiering decisions.
Jira issue: PEBBLE-1320