Quick skim, but I would imagine something like this:

session/
  provider.go # defines interface only and small `New` method maybe, or even skip taht and do all in cmd.
  redis/redis.go # implements ^
  memory/memory.go  # implements ^

and types are not even needed. Basically - no flags provided - provider is memory, if redis-smth-smth is provided - its redis. If 2 provider flags (not yet supported) is there - validate in cmd and fail.

to the point where every provider could have its own AddFlags(), Validate(), etc methods and be loaded into main cmd flag-set

You know that adding this is calling for e2e test? :D could be follow-up but if we add this ,we will need redis in tests