Add some comments for mutual ssl authentication

Author: justcoding121

src/Titanium.Web.Proxy/CertificateHandler.cs

@@ -56,6 +56,14 @@ internal bool ValidateServerCertificate(object sender, SessionEventArgsBase sess
         {
             X509Certificate? clientCertificate = null;
 
+            //TODO: Can we use client certificate from client socket's Sslstream.RemoteCertificate?
+            //Because only the client can provide the correct certificate.
+            //Proxy has no idea about client certificate when its running on a remote machine.
+            //That would mean we need to delay AuthenticateAsServer call with client until we reach this method
+            //and decide right here if we should set SslServerAuthenticationOptions.ClientCertificateRequired = true for clientStream.AuthenticateAsServer call.
+            //Sounds like a very complicated change, but technically possible.
+
+            //fallback to the first client certificate from proxy machine certificate store
             if (acceptableIssuers != null && acceptableIssuers.Length > 0 && localCertificates != null &&
                 localCertificates.Count > 0)
             {
@@ -69,7 +77,9 @@ internal bool ValidateServerCertificate(object sender, SessionEventArgsBase sess
                 }
             }
 
-            if (localCertificates != null && localCertificates.Count > 0)
+            //fallback to the first client certificate from proxy machine certificate store
+            if (clientCertificate == null
+                && localCertificates != null && localCertificates.Count > 0)
             {
                 clientCertificate = localCertificates[0];
             }