feat: allow any slack alertmanager notification to be routed via Slackline

This allows any slack channel receiver to be routed via Slackline.

This follows on from the change in gitlab-com/gl-infra/slackline!39 (merged) to allow Slackline to publish to alternative channels.

Just add useSlackline: slacklineWebhookConfigurations.production or useSlackLine: slacklineWebhookConfigurations.staging to the Slack channel definition to have it route via Slackline.

Depending on whether you choose staging or production, a different instance of slackline may be used.

This MR only changes feed_alerts_staging for now. We can extend this to other slack channels, particularly the team notification channels at a later stage.

---

Generated output

Before

- name: feed_alerts_staging
  slack_configs:
    channel: "#feed_alerts_staging"
    ...

After

- name: feed_alerts_staging
  webhook_configs:
  - http_config:
      bearer_token: staging_secret
    send_resolved: true
    url: http://staging.cloudfunctions.net/alertManagerBridge?channel=feed_alerts_staging

cc @niskhakova