feat(openai): 新增 OpenAI v2 版接口客户端，并实现加解密及签名中间件

test/SKIT.FlurlHttpClient.Wechat.OpenAI.UnitTests/TestCase_ApiExecuteDialogTests.cs

@@ -9,7 +9,7 @@ namespace SKIT.FlurlHttpClient.Wechat.OpenAI.UnitTests
         public async Task TestExecuteSign()
         {
             var request = new Models.SignRequest() { UserId = "TEST_USERID" };
-            var response = await TestClients.Instance.ExecuteSignAsync(request);
+            var response = await TestClients.ChatbotInstance.ExecuteSignAsync(request);
 
             Assert.NotNull(response.Signature);
         }