HTTP redirect service plugin

This feature adds IPv6 support for HTTP redirect. When you use a remote IPv6 HTTP redirect server,you can now configure an HTTP service rule to rewrite the IPv6-DA of incoming HTTP requests on the service router. This ensures that the requests reach the remote HTTP redirect server before being redirected to a captive portal. When one uses a local HTTP redirect server, one can configure an HTTP service rule to redirect HTTP requests to a captive portal within a walled garden.