Support descriptive statistics

[pdurbin]

It looks like on Jan 6 mlcommons/croissant#990 was merged which adds a section to the Croissant spec about supporting descriptive statistics. Here's a preview (draft 1.1 spec): https://docs.mlcommons.org/croissant/docs/croissant-spec-1.1.html#application-representing-descriptive-statistics

Here's the related issue I opened upstream (we tend to use the term "summary statistics":

• summary statistics (mean, max, min, etc.) mlcommons/croissant#640