feat: relay peer end-to-end encryption via Noise IK handshake (#1960)

Enable encryption for non-direct nodes requiring relay forwarding.
When secure_mode is enabled, peers perform Noise IK handshake to
establish an encrypted PeerSession. Relay packets are encrypted at
the sender and decrypted at the receiver. Intermediate forwarding
nodes cannot read plaintext data.

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Co-authored-by: Copilot <198982749+Copilot@users.noreply.github.com>
Co-authored-by: KKRainbow <5665404+KKRainbow@users.noreply.github.com>

easytier/src/peers/mod.rs

@@ -10,6 +10,7 @@ pub mod peer_ospf_route;
 pub mod peer_rpc;
 pub mod peer_rpc_service;
 pub mod peer_session;
+pub mod relay_peer_map;
 pub mod route_trait;
 pub mod rpc_service;