Feature/system.text.json (#192)

Initial support for System.Text.Json and some refactoring

CryptoExchange.Net/Converters/ArrayPropertyAttribute.cs

@@ -0,0 +1,27 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+
+namespace CryptoExchange.Net.Converters
+{
+    /// <summary>
+    /// Mark property as an index in the array
+    /// </summary>
+    [AttributeUsage(AttributeTargets.Property)]
+    public class ArrayPropertyAttribute : Attribute
+    {
+        /// <summary>
+        /// The index in the array
+        /// </summary>
+        public int Index { get; }
+
+        /// <summary>
+        /// ctor
+        /// </summary>
+        /// <param name="index"></param>
+        public ArrayPropertyAttribute(int index)
+        {
+            Index = index;
+        }
+    }
+}