Restore support for custom JSON serializer configuration

Allow external configuration of JSON serializer settings by introducing an optional delegate. This enhances flexibility and enables customization based on specific use cases.

src/clients/Elsa.Api.Client/Helpers/RefitSettingsHelper.cs

@@ -34,6 +34,8 @@ public static class RefitSettingsHelper
         options.Converters.Add(new VersionOptionsJsonConverter());
         options.Converters.Add(new TypeJsonConverter());
         
+        configureJsonSerializerOptions?.Invoke(serviceProvider, options);
+        
         return options;
     }
 }