[#8] Add auto update checker

Elwig/Helpers/HttpClientExtensions.cs

@@ -0,0 +1,24 @@
+using System;
+using System.IO;
+using System.Net.Http;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace Elwig.Helpers {
+    public static class HttpClientExtensions {
+        public static async Task DownloadAsync(this HttpClient client, string requestUri, Stream destination, IProgress<double>? progress = null, CancellationToken cancellationToken = default) {
+            using var response = await client.GetAsync(requestUri, HttpCompletionOption.ResponseHeadersRead, cancellationToken);
+            var contentLength = response.Content.Headers.ContentLength;
+
+            using var download = await response.Content.ReadAsStreamAsync(cancellationToken);
+            if (progress == null || !contentLength.HasValue) {
+                await download.CopyToAsync(destination, cancellationToken);
+                return;
+            }
+
+            var relativeProgress = new Progress<long>(totalBytes => progress.Report((double)totalBytes / contentLength.Value));
+            await download.CopyToAsync(destination, 81920, relativeProgress, cancellationToken);
+            progress.Report(100.0);
+        }
+    }
+}