This permits other programs (in other repos) to conditionally
import ipn/store/awsstore and/or ipn/store/kubestore and have them
register themselves, rather than feature/condregister doing it.
Updates tailscale/corp#32922
Change-Id: I2936229ce37fd2acf9be5bf5254d4a262d090ec1
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>