Use service provider HttpMessageHandler factory for GitHub client

tests/Tgstation.Server.Host.Tests/BasicHttpMessageHandlerFactory.cs

@@ -0,0 +1,19 @@
+using System;
+using System.Net.Http;
+
+namespace Tgstation.Server.Host.Tests
+{
+	/// <summary>
+	/// Basic <see cref="IHttpMessageHandlerFactory"/> implementation for testiong
+	/// </summary>
+	public sealed class BasicHttpMessageHandlerFactory : IHttpMessageHandlerFactory, IDisposable
+	{
+		readonly HttpClientHandler handler = new();
+
+		public HttpMessageHandler CreateHandler(string name)
+			=> handler;
+
+		public void Dispose()
+			=> handler.Dispose();
+	}
+}