Added some util methods, Added CommaSplitStringConverter

CryptoExchange.Net/Converters/SystemTextJson/CommaSplitStringConverter.cs

@@ -0,0 +1,30 @@
+using System;
+using System.Diagnostics.CodeAnalysis;
+using System.Linq;
+using System.Text.Json;
+using System.Text.Json.Serialization;
+
+namespace CryptoExchange.Net.Converters.SystemTextJson
+{
+    /// <summary>
+    /// Converter for comma separated string values
+    /// </summary>
+    public class CommaSplitStringConverter : JsonConverter<string[]>
+    {
+        /// <inheritdoc />
+        public override string[]? Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
+        {
+            var str = reader.GetString();
+            if (string.IsNullOrEmpty(str))
+                return [];
+
+            return str!.Split(',').ToArray() ?? [];
+        }
+
+        /// <inheritdoc />
+        public override void Write(Utf8JsonWriter writer, string[] value, JsonSerializerOptions options)
+        {
+            writer.WriteStringValue(string.Join(",", value));
+        }
+    }
+}