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/awsstore/store_aws_stub.go

@@ -0,0 +1,19 @@
+// Copyright (c) 2021 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.
+
+//go:build !linux
+// +build !linux
+
+package awsstore
+
+import (
+	"fmt"
+	"runtime"
+
+	"tailscale.com/ipn"
+)
+
+func NewStore(string) (ipn.StateStore, error) {
+	return nil, fmt.Errorf("AWS store is not supported on %v", runtime.GOOS)
+}