ipn/store: automatically migrate between plaintext and encrypted state (#16318)

Add a new `--encrypt-state` flag to `cmd/tailscaled`. Based on that
flag, migrate the existing state file to/from encrypted format if
needed.

Updates #15830

Signed-off-by: Andrew Lytvynov <awly@tailscale.com>

ipn/store/mem/store_mem.go

@@ -7,6 +7,7 @@ package mem
 import (
 	"bytes"
 	"encoding/json"
+	"iter"
 	"sync"
 
 	xmaps "golang.org/x/exp/maps"
@@ -85,3 +86,16 @@ func (s *Store) ExportToJSON() ([]byte, error) {
 	}
 	return json.MarshalIndent(s.cache, "", "  ")
 }
+
+func (s *Store) All() iter.Seq2[ipn.StateKey, []byte] {
+	return func(yield func(ipn.StateKey, []byte) bool) {
+		s.mu.Lock()
+		defer s.mu.Unlock()
+
+		for k, v := range s.cache {
+			if !yield(k, v) {
+				break
+			}
+		}
+	}
+}