Added CI&CD pipelines

Changed test project to .net5
Changed ThreadPool to use CancellationTokens

SystemExtensions.Tests/Utils/MockHttpMessageHandler.cs

@@ -0,0 +1,29 @@
+using System;
+using System.Net.Http;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace SystemExtensionsTests.Utils
+{
+    public sealed class MockHttpMessageHandler : HttpMessageHandler
+    {
+        public Func<HttpRequestMessage, HttpResponseMessage> ResponseResolver { get; set; }
+        public Func<HttpRequestMessage, Task<HttpResponseMessage>> ResponseResolverAsync { get; set; }
+
+        public MockHttpMessageHandler()
+        {
+        }
+
+        protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
+        {
+            if (this.ResponseResolver is object)
+            {
+                return Task.FromResult(this.ResponseResolver(request));
+            }
+            else
+            {
+                return this.ResponseResolverAsync(request);
+            }
+        }
+    }
+}