ci: add owners awareness to deployment-notifier

Created by: jhchabran

Adds an additional flag to deployment-notifier to compute the owners for each PR and mentions them posting on Slack.

I haven't touched yet on how to handle team notifications, it's something that the teammateResolver can't handle for now.

Depends on: https://github.com/sourcegraph/codenotify/pull/17

Closes https://github.com/sourcegraph/sourcegraph/issues/33923

Test plan