webhooks: get proxy settings from ini file

closes #7710

pkg/services/notifications/webhook.go

@@ -25,6 +25,7 @@ type Webhook struct {
 }
 
 var netTransport = &http.Transport{
+	Proxy: http.ProxyFromEnvironment,
 	Dial: (&net.Dialer{
 		Timeout: 30 * time.Second,
 	}).Dial,