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.go

@@ -158,6 +158,26 @@ func (s *MemoryStore) WriteState(id StateKey, bs []byte) error {
 	return nil
 }
 
+// LoadFromJSON attempts to unmarshal json content into the
+// in-memory cache.
+func (s *MemoryStore) LoadFromJSON(data []byte) error {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	return json.Unmarshal(data, &s.cache)
+}
+
+// ExportToJSON exports the content of the cache to
+// JSON formatted []byte.
+func (s *MemoryStore) ExportToJSON() ([]byte, error) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	if len(s.cache) == 0 {
+		// Avoid "null" serialization.
+		return []byte("{}"), nil
+	}
+	return json.MarshalIndent(s.cache, "", "  ")
+}
+
 // FileStore is a StateStore that uses a JSON file for persistence.
 type FileStore struct {
 	path string