mod_rest: Add configuration of which stanzas to route to callback Makes it simpler to build APIs that only handle a certain kind of stanzas, letting them be handled by the unhandled stanza handler instead of having to write code to ignore certain kinds of stanzas.

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

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