Fix tunnel communication corruption and add --enc option to wsssht

- Fixed critical data corruption in WebSocket tunnels between wsssht, wssshd, and wssshc
- Root cause: Inconsistent encoding between components (wsssht used base64, wssshc used hex)
- Solution: Implemented consistent hex encoding across all tunnel data transmission
- Added encoding field to tunnel structures for proper encoding negotiation
- Fixed handle_tunnel_data() to decode using correct encoding type instead of guessing

- Added --enc option to wsssht for data encoding control
- --enc hex: Hexadecimal encoding (default, backward compatible)
- --enc base64: Base64 encoding for efficiency
- --enc bin: Direct binary data transmission
- Configuration file support with enc = hex option in wsssht.conf
- Automatic encoding negotiation between wsssht and wssshc clients
- wsssh and wsscp can pass --enc option to ProxyCommand for wsssht

- Updated documentation and examples
- Maintained backward compatibility