Specify Cookie partitionining / CHIPS

We should integrate https://datatracker.ietf.org/doc/draft-cutler-httpbis-partitioned-cookies/00/ into the layered cookies specification. See also the [Cookie layering design doc](https://docs.google.com/document/d/1GFCYZs0arxBRll33SMPAcXYqrJpCVv9XEd5YW_tKNRU/edit?tab=t.0) for integration thoughts.

cc @DCtheTall @annevk @bvandersloot-mozilla