Added ObjectStringConverter for double serialized objects

CryptoExchange.Net/Converters/SystemTextJson/ObjectStringConverter.cs

@@ -0,0 +1,40 @@
+using System;
+using System.Collections.Generic;
+using System.Diagnostics;
+using System.Runtime.Serialization;
+using System.Text;
+using System.Text.Json.Serialization;
+using System.Text.Json;
+using System.Globalization;
+
+namespace CryptoExchange.Net.Converters.SystemTextJson
+{
+    /// <summary>
+    /// 
+    /// </summary>
+    /// <typeparam name="T"></typeparam>
+    public class ObjectStringConverter<T> : JsonConverter<T>
+    {
+        /// <inheritdoc />
+        public override T? Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
+        {
+            if (reader.TokenType == JsonTokenType.Null)
+                return default;
+
+            var value = reader.GetString();
+            if (string.IsNullOrEmpty(value))
+                return default;
+
+            return (T?)JsonDocument.Parse(value).Deserialize(typeof(T));
+        }
+
+        /// <inheritdoc />
+        public override void Write(Utf8JsonWriter writer, T? value, JsonSerializerOptions options)
+        {
+            if (value is null)
+                writer.WriteStringValue("");
+
+            writer.WriteStringValue(JsonSerializer.Serialize(value, options));
+        }
+    }
+}