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_test.go

@@ -5,6 +5,7 @@ package ipn
 
 import (
 	"bytes"
+	"iter"
 	"sync"
 	"testing"
 
@@ -31,6 +32,19 @@ func (s *memStore) WriteState(k StateKey, v []byte) error {
 	return nil
 }
 
+func (s *memStore) All() iter.Seq2[StateKey, []byte] {
+	return func(yield func(StateKey, []byte) bool) {
+		s.mu.Lock()
+		defer s.mu.Unlock()
+
+		for k, v := range s.m {
+			if !yield(k, v) {
+				break
+			}
+		}
+	}
+}
+
 func TestWriteState(t *testing.T) {
 	var ss StateStore = new(memStore)
 	WriteState(ss, "foo", []byte("bar"))