Implement end-to-end encryption for core-web connections using the
Noise protocol framework with the following changes:
Client-side (easytier/src/web_client/):
- Add security.rs module with Noise handshake implementation
- Add upgrade_client_tunnel() for client-side handshake
- Add Noise frame encryption/decryption via TunnelFilter
- Integrate GetFeature RPC for capability negotiation
- Support secure_mode option to enforce encrypted connections
- Handle graceful fallback for backward compatibility
Server-side (easytier-web/):
- Accept Noise handshake in client_manager
- Expose encryption support via GetFeature RPC
The implementation uses Noise_NN_25519_ChaChaPoly_SHA256 pattern for
encryption without authentication. Provides backward compatibility
with automatic fallback to plaintext connections.