Describe-consumer admin should not return unstable membership information

When a consumer group is in the “prepare-rebalance” phase, it's unclear if all its currently registered members would still be re-joining in the new generation or not, in this case, if we simply return the current members map to the describe-consumer request it may be misleading as users would be getting spurious results that may contain those dropping or even zombie consumers.

So I think during the prepare-rebalance phase, we should either only return members who's join-group requests have already been received, OR we simply return the response with no members and indicate that via prepare-rebalance state the membership info is unstable and hence won't be returned.