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/kubestore/store_kube.go

@@ -7,6 +7,7 @@ package kubestore
 import (
 	"context"
 	"fmt"
+	"iter"
 	"log"
 	"net"
 	"os"
@@ -428,3 +429,7 @@ func sanitizeKey[T ~string](k T) string {
 		return '_'
 	}, string(k))
 }
+
+func (s *Store) All() iter.Seq2[ipn.StateKey, []byte] {
+	return s.memory.All()
+}