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.
No, I guess that's fine. But if it's just for backwards compatibility, don't you cover that with your `value["version"] == 1` case, and so you don't need it also in the constructor?