mod_external_services: Factor out public function for converting to XML Along with the previous commit, allows building the XML thing yourself, should you wish to send it yourself or use it in a different context than an iq reply. API change: The 'reply' is removed from the event.

This module lets you block connections to any remote servers not on a
whitelist.

``` {.lua}
s2s_whitelist = {
    "example.org",
}
```