Added nullable int converter for System.Text.Json

CryptoExchange.Net/Converters/SystemTextJson/IntConverter.cs

@@ -0,0 +1,40 @@
+using System;
+using System.Globalization;
+using System.Text.Json;
+using System.Text.Json.Serialization;
+
+namespace CryptoExchange.Net.Converters.SystemTextJson
+{
+    /// <summary>
+    /// Int converter
+    /// </summary>
+    public class IntConverter : JsonConverter<int?>
+    {
+        /// <inheritdoc />
+        public override int? Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
+        {
+            if (reader.TokenType == JsonTokenType.Null)
+                return null;
+
+            if (reader.TokenType == JsonTokenType.String)
+            {
+                var value = reader.GetString();
+                if (string.IsNullOrEmpty(value))
+                    return null;
+
+                return int.Parse(value, NumberStyles.Integer, CultureInfo.InvariantCulture);
+            }
+
+            return reader.GetInt32();
+        }
+
+        /// <inheritdoc />
+        public override void Write(Utf8JsonWriter writer, int? value, JsonSerializerOptions options)
+        {
+            if (value == null)
+                writer.WriteNullValue();
+            else
+                writer.WriteNumberValue(value.Value);
+        }
+    }
+}

CryptoExchange.Net/Converters/SystemTextJson/SerializerOptions.cs

@@ -21,6 +21,7 @@ public static class SerializerOptions
                         new EnumConverter(),
                         new BoolConverter(),
                         new DecimalConverter(),
+                        new IntConverter()
                     }
         };
     }