ipn/store, feature/condregister: permit callers to empty import optonal ipn stores

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>

ipn/store/kubestore/store_kube.go

@@ -16,6 +16,7 @@ import (
 
 	"tailscale.com/envknob"
 	"tailscale.com/ipn"
+	"tailscale.com/ipn/store"
 	"tailscale.com/ipn/store/mem"
 	"tailscale.com/kube/kubeapi"
 	"tailscale.com/kube/kubeclient"
@@ -25,6 +26,13 @@ import (
 	"tailscale.com/util/mak"
 )
 
+func init() {
+	store.Register("kube:", func(logf logger.Logf, path string) (ipn.StateStore, error) {
+		secretName := strings.TrimPrefix(path, "kube:")
+		return New(logf, secretName)
+	})
+}
+
 const (
 	// timeout is the timeout for a single state update that includes calls to the API server to write or read a
 	// state Secret and emit an Event.