mod_http_oauth2: Validate consistency of response and grant types Ensure that these correlated fields make sense per RFC 7591 § 2.1, even though we currently only check the response type during authorization. This could probably all be deleted if (when!) we remove the implicit grant, since then these things don't make any sense anymore.

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

``` {.lua}
modules_enabled = {
    -- other modules --
    "s2s_whitelist",

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