refactor(tenpayv2): 优化签名生成逻辑

src/SKIT.FlurlHttpClient.Wechat.OpenAI/Utilities/Internal/JwtUtility.cs

@@ -0,0 +1,25 @@
+using System;
+using JWT;
+using JWT.Algorithms;
+using JWT.Serializers;
+using Newtonsoft.Json;
+
+namespace SKIT.FlurlHttpClient.Wechat.OpenAI.Utilities
+{
+    internal static class JwtUtility
+    {
+        private static readonly Lazy<IJwtEncoder> _encoder = new Lazy<IJwtEncoder>(() =>
+        {
+            IJwtAlgorithm algorithm = new HMACSHA256Algorithm();
+            IJsonSerializer serializer = new JsonNetSerializer(new JsonSerializer() { NullValueHandling = NullValueHandling.Ignore });
+            IBase64UrlEncoder urlEncoder = new JwtBase64UrlEncoder();
+            IJwtEncoder encoder = new JwtEncoder(algorithm, serializer, urlEncoder);
+            return encoder;
+        }, isThreadSafe: true);
+
+        public static string EncodeWithHS256(object payload, string secret)
+        {
+            return _encoder.Value.Encode(payload, secret);
+        }
+    }
+}