Introduce secure mode (part 1) (#1808)

Use noise protocol on handshake. Check peer's public key if needed. Also support rekey and replay attack prevention.

E2EE and temporary password will be implemented based on this.

easytier/src/proto/common.proto

@@ -230,3 +230,13 @@ message LimiterConfig {
   optional uint64 fill_duration_ms =
       3; // default 10ms, the period to fill the bucket
 }
+
+message SecureModeConfig {
+  bool enabled = 1; 
+
+  // base64(X25519 private key), used by shared node to present a stable identity
+  optional string local_private_key = 2;
+
+  // base64(X25519 public key), required if local_private_key is set
+  optional string local_public_key = 3;
+}