Implement AuthService and ApiClient core logic

Models/ChatModels.cs

@@ -0,0 +1,24 @@
+using Chtn.CSharp.SDK.Models.Common;
+using Newtonsoft.Json;
+using System.Collections.Generic;
+
+namespace Chtn.CSharp.SDK.Models.Chat
+{
+    public class Chat
+    {
+        [JsonProperty("chatid")] public string ChatId { get; set; }
+        [JsonProperty("displayName")] public string DisplayName { get; set; }
+        [JsonProperty("latestMessage")] public LatestMessage Latest { get; set; }
+    }
+
+    public class Message
+    {
+        [JsonProperty("msgid")] public string MsgId { get; set; }
+        [JsonProperty("author")] public string Author { get; set; }
+        [JsonProperty("message")] public string Content { get; set; }
+        [JsonProperty("sent_at")] public TimeStamp SentAt { get; set; }
+        [JsonProperty("files")] public List<Attachment> Files { get; set; }
+    }
+
+    public class LatestMessage { public string Message { get; set; } public string MsgId { get; set; } }
+}