modify CapOptions Extension to support multiple extensions.

src/DotNetCore.CAP/CAP.Options.cs

 using System;
+using System.Collections.Generic;
 
 namespace DotNetCore.CAP
 {
@@ -7,7 +8,7 @@ namespace DotNetCore.CAP
     /// </summary>
     public class CapOptions
     {
-        internal ICapOptionsExtension Extension { get; private set; }
+        internal IList<ICapOptionsExtension> Extensions { get; private set; }
 
         /// <summary>
         /// Default value for polling delay timeout, in seconds.
@@ -23,6 +24,7 @@ namespace DotNetCore.CAP
         {
             CronExp = DefaultCronExp;
             PollingDelay = DefaultPollingDelay;
+            Extensions = new List<ICapOptionsExtension>();
         }
 
         /// <summary>
@@ -41,7 +43,10 @@ namespace DotNetCore.CAP
 		/// <param name="extension"></param>
 		public void RegisterExtension(ICapOptionsExtension extension)
         {
-            Extension = extension ?? throw new ArgumentNullException(nameof(extension));
+            if (extension == null)
+                throw new ArgumentNullException(nameof(extension));
+
+            Extensions.Add(extension);
         }
     }
 }
\ No newline at end of file

src/DotNetCore.CAP/CAP.ServiceCollectionExtensions.cs

@@ -53,10 +53,13 @@ namespace Microsoft.Extensions.DependencyInjection
             services.AddSingleton<IQueueExecutorFactory, QueueExecutorFactory>();
             services.AddSingleton<IQueueExecutor, SubscibeQueueExecutor>();
 
-            //Options
+            //Options and extension service
             var options = new CapOptions();
             setupAction(options);
-            options.Extension?.AddServices(services);
+            foreach (var serviceExtension in options.Extensions)
+            {
+                serviceExtension.AddServices(services);
+            }          
             services.AddSingleton(options);
 
             return new CapBuilder(services);