Add legacy ASP.NET Core 3.1 version of this app

AS1024.NetworkQuality.Server.NetCore31/Utils/SnakeCaseNamingPolicy.cs

@@ -0,0 +1,31 @@
+using System;
+using System.Text.Json;
+
+public class SnakeCaseNamingPolicy : JsonNamingPolicy
+{
+    public override string ConvertName(string name)
+    {
+        return string.IsNullOrEmpty(name) ? name : ToSnakeCase(name);
+    }
+
+    private string ToSnakeCase(string name)
+    {
+        var stringBuilder = new System.Text.StringBuilder();
+        for (var i = 0; i < name.Length; i++)
+        {
+            if (char.IsUpper(name[i]))
+            {
+                if (i > 0)
+                {
+                    stringBuilder.Append('_');
+                }
+                stringBuilder.Append(char.ToLowerInvariant(name[i]));
+            }
+            else
+            {
+                stringBuilder.Append(name[i]);
+            }
+        }
+        return stringBuilder.ToString();
+    }
+}