Let the .NET core/standard WebHostBuilder use a random port (#417)

* wip

* code reformat

src/WireMock.Net/Owin/HostUrlOptions.cs

@@ -0,0 +1,41 @@
+using System.Collections.Generic;
+using WireMock.Util;
+
+namespace WireMock.Owin
+{
+    internal class HostUrlOptions
+    {
+        public ICollection<string> Urls { get; set; }
+
+        public bool UseSSL { get; set; }
+
+        public ICollection<(string Url, int Port)> GetDetails()
+        {
+            var list = new List<(string Url, int Port)>();
+            if (Urls == null)
+            {
+                int port = FindFreeTcpPort();
+                list.Add(($"{(UseSSL ? "https" : "http")}://localhost:{port}", port));
+            }
+            else
+            {
+                foreach (string url in Urls)
+                {
+                    PortUtils.TryExtract(url, out string protocol, out string host, out int port);
+                    list.Add((url, port));
+                }
+            }
+
+            return list;
+        }
+
+        private int FindFreeTcpPort()
+        {
+#if USE_ASPNETCORE || NETSTANDARD2_0
+            return 0;
+#else
+            return PortUtils.FindFreeTcpPort();
+#endif
+        }
+    }
+}