Markdown support for Slack message

This is already possible. The message_body field is already just simply passed directly to Slack. I have this in our slack notification policy, which includes bold text (*foo*)and a link (<url|text>):

message_body := sprintf(
  concat("\n", [
    "<%s|Drift was detected in %s>",
    concat(", ", [msg |
      some action in action_summary_order
      msg := object.get(resource_counts, action, null)
      msg != null
    ]),
  ]), 
  [run_url, stack.name],
) if {
  drift_detection_run
  run.state == "FINISHED"
  run.type == "PROPOSED"
}