Initial mirror.

MareSynchronosAPI/Data/SignedChatMessage.cs

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+using MessagePack;
+
+namespace MareSynchronos.API.Data;
+
+[MessagePackObject(keyAsPropertyName: true)]
+public record SignedChatMessage(ChatMessage Message, UserData Sender) : ChatMessage(Message)
+{
+    // Sender and timestamp are set by the server
+    public UserData Sender { get; set; } = Sender;
+    public long Timestamp { get; set; } = 0;
+    // Signature is generated by the server as SHA256(Sender.UID | Timestamp | Destination | Message)
+    // Where Destination is either the receiver's UID, or the group GID
+    public string Signature { get; set; } = string.Empty;
+}