ipn/store: add common package for instantiating ipn.StateStores

Also move KubeStore and MemStore into their own package.

RELNOTE: tsnet now supports providing a custom ipn.StateStore.

Signed-off-by: Maisem Ali <maisem@tailscale.com>

ipn/store/stores_linux.go

@@ -0,0 +1,26 @@
+// Copyright (c) 2022 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package store
+
+import (
+	"strings"
+
+	"tailscale.com/ipn"
+	"tailscale.com/ipn/store/awsstore"
+	"tailscale.com/ipn/store/kubestore"
+	"tailscale.com/types/logger"
+)
+
+func init() {
+	registerAvailableExternalStores = registerExternalStores
+}
+
+func registerExternalStores() {
+	Register("kube:", func(logf logger.Logf, path string) (ipn.StateStore, error) {
+		secretName := strings.TrimPrefix(path, "kube:")
+		return kubestore.New(logf, secretName)
+	})
+	Register("arn:", awsstore.New)
+}