Implement AuthService and ApiClient core logic

Models/SessionModels.cs

@@ -0,0 +1,12 @@
+using Chtn.CSharp.SDK.Models.User;
+using Newtonsoft.Json;
+
+namespace Chtn.CSharp.SDK.Models.Session
+{
+    public class Session
+    {
+        [JsonProperty("userData")] public PersonalUserData UserData { get; set; }
+        [JsonProperty("token")] public string Token { get; set; }
+    }
+    public class ValidateSessionResp { public bool ValidationOk { get; set; } }
+}