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>

cmd/tsconnect/src/lib/js-state-store.ts

@@ -10,4 +10,7 @@ export const sessionStateStorage: IPNStateStorage = {
   getState(id) {
     return window.sessionStorage[`ipn-state-${id}`] || ""
   },
+  all() {
+    return JSON.stringify(window.sessionStorage)
+  },
 }