synchronize ICallService with stateful implementation and resolve build errors

Services/CallService.cs

@@ -0,0 +1,45 @@
+using System;
+using System.Threading.Tasks;
+using Chtn.CSharp.SDK.Core;
+using Chtn.CSharp.SDK.Models.Call;
+
+namespace Chtn.CSharp.SDK.Services
+{
+    public interface ICallService
+    {
+        Task InviteToCall();
+        Task<GetRTCAccessResp> GetRTCAccess();
+    }
+
+    public class CallService: ICallService
+    {
+        private readonly ApiClient _apiClient;
+        private readonly string _userId;
+        private readonly string _token;
+
+        public CallService(ApiClient apiClient, string userId, string token)
+        {
+            _apiClient = apiClient ?? throw new ArgumentNullException(nameof(apiClient));
+            _userId = userId;
+            _token = token;
+        }
+
+        public async Task InviteToCall()
+        {
+            var body = new
+            {
+                userid = _userId
+            };
+
+            await _apiClient.PostAsync<object, object>("v2/chat/rtcInvite", body);
+        }
+        public async Task<GetRTCAccessResp> GetRTCAccess()
+        {
+            var body = new
+            {
+                userid = _userId
+            };
+            return await _apiClient.PostAsync<object, GetRTCAccessResp>("v2/chat/getRTCAccess", body);
+        }
+    }
+}