Support secrets in Alert Channels

We’d like to use a secret in an alert channel HTTP header so we don’t have to hardcode the secret in our Checkly git repo.

Here’s how we’d envision we’d use it:

new WebhookAlertChannel("rootly-p2", {
  name: "Rootly P2",
  method: "POST",
  url: new URL(
    "https://webhooks.rootly.com/webhooks/incoming/checkly_webhooks"
  ),
  headers: [{ key: "secret", value: "{{ ROOTLY_WEBHOOK_SECRET }}" }],
});

Right now it recognizes the template but just replaces the value with an empty string leaving the header with no value.