add the classification extension for Palette (colormap) values

https://github.com/stac-extensions/classification

For now it's still at Pilot stage so maybe we wait for `Candidate`

Read the linked STAC classification extension specification first, paying attention to its Candidate status. Then inspect how rio-stac represents Palette (colormap) values and handles existing STAC extensions; the work is done when classification metadata is supported without proceeding against the stated Pilot-stage constraint.

Written by the indexing model from the issue text.