Feature/system.text.json (#192)

Initial support for System.Text.Json and some refactoring

CryptoExchange.Net/Converters/SystemTextJson/DecimalConverter.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>
+    /// Decimal converter
+    /// </summary>
+    public class DecimalConverter : JsonConverter<decimal?>
+    {
+        /// <inheritdoc />
+        public override decimal? 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 decimal.Parse(value, NumberStyles.Float, CultureInfo.InvariantCulture);
+            }
+
+            return reader.GetDecimal();
+        }
+
+        /// <inheritdoc />
+        public override void Write(Utf8JsonWriter writer, decimal? value, JsonSerializerOptions options)
+        {
+            if (value == null)
+                writer.WriteNullValue();
+            else
+                writer.WriteNumberValue(value.Value);
+        }
+    }
+}