feat: Add custom headers support for Webhook AlertMethod

[kskitek]

Motivation

Webhook alert method does not support custom HTTP headers now.

Summary

• the headers in .tf are split into headers and sensitive_headers to allow settings secrets..

resource "nobl9_alert_method_webhook" "webhook" {
  name            = "webhook"
  project         = "default"
  url             = "http://web.net"
  template        = "SLO needs attention $slo_name"
  headers         = {
    "X-Custom-Header" = "custom value"
  }
  sensitive_headers = {
    "Authorization" = "Bearer xyz"
  }
}

Release Notes

Webhook alert method now supports custom HTTP headers.