chore(tenpayv3): 新增多租户证书平台管理器的示例代码

samples/SKIT.FlurlHttpClient.Wechat.TenpayV3.Sample_Net5/Program.cs

@@ -1,8 +1,4 @@
-using System;
-using System.Collections.Generic;
-using System.Linq;
 using System.Text;
-using System.Threading.Tasks;
 using Microsoft.AspNetCore.Hosting;
 using Microsoft.Extensions.Hosting;