added stub for an InjectableTcpRpcClient

creating a TcpClient, performing the Handshake and then injecting the stream into the RpcClient could be a viable workaround to the internal stream of the default RpcClient being inaccessible

FabAccessAPI/InjectableTcpRpcClient.cs

@@ -5,6 +5,8 @@ using Capnp.Rpc;
 
 namespace FabAccessAPI {
     class InjectableTcpRpcClient : TcpRpcClient {
-
+        public InjectableTcpRpcClient() {
+            throw new NotImplementedException();
+        }
     }
 }