feat: add Hex/Base64 typed bytes support for flags, env, and config

[leodido]

Summary

• wire type-based byte semantics into built-in hooks:
  • []byte -> raw textual bytes
  • structcli.Hex -> hex-decoded bytes
  • structcli.Base64 -> base64-decoded bytes
• register corresponding define hooks and decode hooks so flags/env/config/defaults all resolve consistently
• add end-to-end hook tests covering defaults, flags, env, config, precedence, and invalid hex input

Testing

• GOCACHE=/tmp/structcli-gocache go test ./internal/hooks -count=1
• GOCACHE=/tmp/structcli-gocache go test $(go list ./... | grep -v 'github.com/leodido/structcli/internal/scope') -count=1