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_ApiExecuteTokenTests.cs

@@ -0,0 +1,17 @@
+using System.Threading.Tasks;
+using Xunit;
+
+namespace SKIT.FlurlHttpClient.Wechat.OpenAI.UnitTests
+{
+    public class TestCase_ApiExecuteTokenTests
+    {
+        [Fact(DisplayName = "测试用例：调用 API [POST] /v2/token")]
+        public async Task TestExecuteTokenV2()
+        {
+            var request = new Models.TokenV2Request() { Account = "TEST_ACCOUNTID" };
+            var response = await TestClients.OpenAIInstance.ExecuteTokenV2Async(request);
+
+            Assert.NotNull(response.Data?.AccessToken);
+        }
+    }
+}