Generic HTTP webhook delivery backend.

This notifier sends the formatted message to any HTTP endpoint using Req. It is the most flexible of the built-in delivery modules because it allows the HTTP method, headers, and JSON payload shape to be customized.

Required config:

• :webhook_url - the destination URL

Optional config:

• :method - HTTP verb, defaults to :post
• :headers - a list of request headers
• :payload - a base JSON map merged with %{text: message}

Retry behavior is shared with Slack and Teams through NatureWhistle.Notifier.Retry.