Adds named pipes access for windows to security policy (elastic/elasticsearch#4927)

Windows requires named pipes to be created at a specific location so we need to allow access to this special location in the security policy for the machine learning plugin to run on X-Pack

Original commit: elastic/x-pack-elasticsearch@3079e88713

elasticsearch/src/main/plugin-metadata/plugin-security.policy

@@ -27,4 +27,7 @@ grant {
 
   // needed for multiple server implementations used in tests
   permission java.net.SocketPermission "*", "accept,connect";
+
+  // needed for Windows named pipes in machine learning
+  permission java.io.FilePermission "\\\\.\\pipe\\*", "read,write";
 };