add producer.

src/Cap.Consistency/Producer/IProducerClient.cs

 using System;
 using System.Collections.Generic;
 using System.Text;
+using System.Threading.Tasks;
 
 namespace Cap.Consistency.Producer
 {
     public interface IProducerClient
     {
-
-
-
+        Task SendAsync(string topic, string content);
     }
 }