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 think if you implement `__hash__`, you need to implement `__eq__`, but I'm not sure if the reverse is true. But do you need to compare scheme objects for equality, anyway?