Add Custom HTTP Headers (Authorization) to Webhook Integrations#1520

Please add support for custom HTTP headers (name/value) in UptimeRobot Webhook integrations.Many destinations require authenticated requests via headers, e.g. Zoom Team Chat Incoming Webhook requires:
Authorization: <verification token>
(Zoom doc: KB0067640)Right now, webhook integrations only allow customizing POST values and sending data as query string / POST parameters / JSON, but there is no way to set headers. This forces users to run a proxy/relay service just to add Authorization, which adds cost, complexity, and a new failure point.Acceptance criteria:
Allow adding one or more headers (name/value) per webhook integration.
Must support Authorization header.
Safe handling of secrets (masked in UI, not retrievable after save; stored encrypted).
Keep existing templating/variables and payload options unchanged.
Example use case (Zoom):
URL: https://integrations.zoom.us/chat/webhooks/incomingwebhook/<id>?format=message
Header: Authorization: <token>
Body: plain text message