Make ServiceChannelProxy implement IAsyncDisposable

So that channels created through a `ChannelFactory<T>` without inheriting from `ClientBase<T>` can also benefit from the asynchronous disposal improvements introduced in dotnet#4865.

Fixes dotnet#5270

src/System.ServiceModel.Primitives/src/System/ServiceModel/Channels/ServiceChannelProxy.cs

@@ -15,7 +15,7 @@
 
 namespace System.ServiceModel.Channels
 {
-    public class ServiceChannelProxy : DispatchProxy, ICommunicationObject, IChannel, IClientChannel, IOutputChannel, IRequestChannel, IServiceChannel, IDuplexContextChannel
+    public class ServiceChannelProxy : DispatchProxy, ICommunicationObject, IChannel, IClientChannel, IOutputChannel, IRequestChannel, IServiceChannel, IDuplexContextChannel, IAsyncDisposable
     {
         private const String activityIdSlotName = "E2ETrace.ActivityID";
         private Type _proxiedType;
@@ -673,6 +673,11 @@ void IDisposable.Dispose()
             ((IClientChannel)_serviceChannel).Dispose();
         }
 
+        ValueTask IAsyncDisposable.DisposeAsync()
+        {
+            return ((IAsyncDisposable)_serviceChannel).DisposeAsync();
+        }
+
         bool IContextChannel.AllowOutputBatching
         {
             get