Added WebRTC stuff - Needs Testing.

RelayClient/Models/RtcSignalMessage.cs

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+namespace RelayClient.Models;
+
+public class RtcSignalMessage
+{
+    public required string Type { get; set; } // rtc_offer / rtc_answer / rtc_ice_candidate / rtc_call_request / rtc_call_accept / rtc_call_reject
+    public required string From { get; set; }
+    public required string To { get; set; }
+
+    public string? Sdp { get; set; }
+    public string? Candidate { get; set; }
+    public string? SdpMid { get; set; }
+    public int? SdpMLineIndex { get; set; }
+}

RelayClient/Models/SocketRtcSignalMessage.cs

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+namespace RelayClient.Models;
+
+public class SocketRtcSignalMessage
+{
+    public required string Type { get; set; } // encrypted_rtc_signal
+    public required string SenderUsername { get; set; }
+    public required string RecipientUsername { get; set; }
+
+    public required string CipherText { get; set; }
+    public required string Nonce { get; set; }
+    public required string Tag { get; set; }
+    public required string EncryptedKey { get; set; }
+}