Added a Downloader for AoC puzzle inputs

AdventOfCode.Downloader/Program.cs

@@ -0,0 +1,34 @@
+// See https://aka.ms/new-console-template for more information
+
+if (args.Length < 3)
+{
+    Console.WriteLine("Usage: AdventOfCode-Downloader.exe <year> <day> <session-cookie>");
+    return;
+}
+
+int year, day;
+if (!(int.TryParse(args[0], out year) && year <= DateTime.Now.Year && year > 2015) || !(int.TryParse(args[1], out day) && day is >= 1 and <= 25))
+{
+    Console.WriteLine("Invalid year or date argument.\nYear must be between 2015 and current year and day must be between 1 and 25");
+    return;
+}
+
+var path = Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.MyDocuments), "AoC-PuzzleInputs", 
+    year.ToString(), "Prod", $"day{day:00}.txt");
+if(!Directory.Exists(Path.GetDirectoryName(path)))
+    Directory.CreateDirectory(Path.GetDirectoryName(path)!);
+
+var url = new Uri($"https://adventofcode.com/{year}/day/{day}/input");
+using var client = new HttpClient();
+client.DefaultRequestHeaders.Add("Cookie", $"session={args[2]}");
+var responseMessage = await client.GetAsync(url);
+if (responseMessage.IsSuccessStatusCode)
+{
+    var inputString = await responseMessage.Content.ReadAsStringAsync();
+    await File.WriteAllTextAsync(path, inputString);
+}
+else
+{
+    var inputString = await responseMessage.Content.ReadAsStringAsync();
+    Console.WriteLine($"Response status code does not indicate Success: {responseMessage.StatusCode} ({responseMessage.StatusCode.ToString()})\nError Message: " + inputString);
+}