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/ipnlocal/state_test.go

@@ -1013,17 +1013,13 @@ func TestEditPrefsHasNoKeys(t *testing.T) {
 }
 
 type testStateStorage struct {
-	mem     mem.Store
+	mem.Store
 	written atomic.Bool
 }
 
-func (s *testStateStorage) ReadState(id ipn.StateKey) ([]byte, error) {
-	return s.mem.ReadState(id)
-}
-
 func (s *testStateStorage) WriteState(id ipn.StateKey, bs []byte) error {
 	s.written.Store(true)
-	return s.mem.WriteState(id, bs)
+	return s.Store.WriteState(id, bs)
 }
 
 // awaitWrite clears the "I've seen writes" bit, in prep for a future