ipn/store/aws, cmd/tailscaled: add AWS SSM ipn.StateStore implementation

From https://github.com/tailscale/tailscale/pull/1919 with
edits by bradfitz@.

This change introduces a new storage provider for the state file. It
allows users to leverage AWS SSM parameter store natively within
tailscaled, like:

    $ tailscaled --state=arn:aws:ssm:eu-west-1:123456789:parameter/foo

Known limitations:
- it is not currently possible to specific a custom KMS key ID

RELNOTE=tailscaled on Linux supports using AWS SSM for state

Edits-By: Brad Fitzpatrick <bradfitz@tailscale.com>
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Signed-off-by: Maxime VISONNEAU <maxime.visonneau@gmail.com>

ipn/store/aws/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 aws
+
+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)
+}