support plumbing for webhooks

[jimdigriz]

To avoid the polling and instead get more instant/live data, we can look to use webhooks instead.

I have only not implemented it as this is very site specific (you need to expose an HTTP server that can talk to your RADIUS server(s) and prompt them to reload) and I would rather have any implementation guided by an actual deployment rather than what I would do.

For those reading this, some of the 'reload' work is in place in the form of the $SIG{'HUP'} hooks throughout the code but we would need to trigger this via radtest as rlm_perl masks out signals.