implement CallService and WebRTC access support

Services/CallServiceMethods.cs

@@ -0,0 +1,29 @@
+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(InviteToCallReq req);
+        Task<GetRTCAccessResp> GetRTCAccess(GetRTCAccessReq req);
+    }
+
+    public class CallServiceProvider: ICallService
+    {
+        private readonly ApiClient _apiClient;
+
+        public CallServiceProvider(ApiClient apiClient)
+        {
+            _apiClient = apiClient ?? throw new ArgumentNullException(nameof(apiClient));
+        }
+
+        public async Task InviteToCall(InviteToCallReq req) =>
+            await _apiClient.PostAsync<InviteToCallReq, object>("v2/chat/rtcInvite", req);
+
+        public async Task<GetRTCAccessResp> GetRTCAccess(GetRTCAccessReq req) =>
+            await _apiClient.PostAsync<GetRTCAccessReq, GetRTCAccessResp>("v2/chat/getRTCAccess", req);
+    }
+}