Coordinated pipelines should post a Slack notification when they start

I've realized recently that we don't really have visibility into when a new coordinated pipeline begins.

We know when a package gets tagged, and when a deploy starts, but we're missing the middle. Also as the coordinated pipeline grows into its intended role of being the single source of truth for a rollout, it will become more important to know where the pipeline is.

I think we should add a second job to the auto_deploy:start stage that just sends a Slack notification to #f_upcoming_release with the CI_PIPELINE_URL.