Add base support for cohorted group configurations

TNL-649

Introduced a new scheme field to user partitions that controls how a user gets assigned
to one of the partitions groups. The default scheme is random, but the next step will be
to introduce a new scheme that will assign the user based upon the cohort they belong to.
i notice this is defined twice in different places - it's small and not a major issue for me, but would be nice to consolidate if low-effort.