enable TLS client forwarding

Copys the incoming TLS client certificate to the outgoing
request. The backend can then use this certificate for
client authentication ie. k8s client cert authentication

docs/user-guide/examples.md

@@ -89,6 +89,7 @@ entryPoint = "https"
   [frontends.frontend2]
   backend = "backend1"
   passHostHeader = true
+  passTLSCert = true
   entrypoints = ["https"] # overrides defaultEntryPoints
     [frontends.frontend2.routes.test_1]
     rule = "Host:{subdomain:[a-z]+}.localhost"