net.http.server: Add new API to get HTTP request from a connection

This information is sometimes necessary in the context where we have a
connection that we know (or believe to be) associated with an incoming HTTP
request.

For example, it can be used to retrieve the IP address of a request (which may
differ from the IP address of the connection, due to X-Forwarded-For and co).

Thanks to the Jitsi team for highlighting this gap in the API.

net/http/server.lua

@@ -428,6 +428,10 @@ end
 function _M.set_option(name, value)
 	options[name] = value;
 end
+function _M.get_request_from_conn(conn)
+	local response = conn and conn._http_open_response;
+	return response and response.request or nil;
+end
 
 _M.listener = listener;
 _M.codes = codes;