Allow to set custom header in System Webhooks

Problem

Currently the System Webhooks don't allow custom header in the configurations, differently from the automation webhook.

Suggested Solution

Add to the System Webhook configuration the ability to set custom headers.

Why This Is Important

It can help external systems to triage and validate messages coming from the webhooks. Also enable the possibility to authenticate in the external system.

Workaround

Use Send web request in Automation since it allows custom headers.

Another possible workaround is to send the headers as params in the URL, but then the external must allow and handle it properly.