Add http(s) proxy support

[dbluxo]

To send notifications behind a proxy, we would need the ability to configure a http(s) proxy.

{"level":"error","ts":"2020-10-16T14:59:37.134Z","logger":"event-server","msg":"failed to send notification","object":"gotk-system/example-app","kind":"Kustomization","error":"postMessage failed: POST https://webhook-url giving up after 5 attempt(s): Post \"https://webhook-url\": context deadline exceeded (Client.Timeout exceeded while awaiting headers)"}