Add `--staging` flag to broadcast command
https://gitlab.com/gitlab-com/chatops/-/blob/master/lib/chatops/commands/broadcast.rb is awesome, but it only works on GitLab.com
I would love to be able to also set broadcast messages on staging, for example in https://gitlab.com/gitlab-com/gl-infra/production/-/issues/7111 having this would have allowed CMOCs to test broadcast messages prior to moving to .com