implement FileTransfer and UserService modules with WebRTC support

Interfaces/IFileWriterAPI.cs

@@ -0,0 +1,13 @@
+using System.Threading.Tasks;
+
+namespace Chtn.CSharp.SDK.Interfaces
+{
+    public interface IFileWriterAPI
+    {
+        Task OpenFile(string transferId, string fileName, long totalSize);
+
+        Task WriteChunk(string transferId, byte[] data);
+
+        void CloseFile(string transferId);
+    }
+}