Add client certificate support (#862)

* Add client certificate support

* Add missing test certificate file

* Review fixes

* Review fixes

* Review fixes

* Review fixes

src/WireMock.Net.Abstractions/Types/ClientCertificateMode.cs

@@ -0,0 +1,31 @@
+namespace WireMock.Types;
+
+#if NETSTANDARD1_3_OR_GREATER || NET461
+/// <summary>
+/// Describes the client certificate requirements for a HTTPS connection.
+/// This enum is the same as https://learn.microsoft.com/en-us/dotnet/api/microsoft.aspnetcore.server.kestrel.https.clientcertificatemode
+/// </summary>
+public enum ClientCertificateMode
+{
+    /// <summary>
+    /// A client certificate is not required and will not be requested from clients.
+    /// </summary>
+    NoCertificate,
+
+    /// <summary>
+    /// A client certificate will be requested; however, authentication will not fail if a certificate is not provided by the client.
+    /// </summary>
+    AllowCertificate,
+
+    /// <summary>
+    /// A client certificate will be requested, and the client must provide a valid certificate for authentication to succeed.
+    /// </summary>
+    RequireCertificate,
+
+    /// <summary>
+    /// A client certificate is not required and will not be requested from clients at the start of the connection.
+    /// It may be requested by the application later.
+    /// </summary>
+    DelayCertificate,
+}
+#endif