Add push mirror status check

What does this MR do?

Adds a script to be scheduled to run daily to check for failures in push mirroring.

Previously, we had an incident which needed an update to pipeline-common to be mirrored to our live environments to unblock deployments. This revealed that the mirrors had been failing silently for weeks.

More context:

• https://gitlab.com/gitlab-org/quality/runbooks/-/merge_requests/5
• gitlab-org/release/tasks#6889 (comment 1607853992)

Script testing results:

• Passing example: https://gitlab.com/gitlab-org/quality/staging/-/pipelines/1067295922
• Failing example: https://gitlab.com/gitlab-org/quality/staging/-/pipelines/1067358961

Sample Slack notification:

Check-list

• Ensure commits include Changelog: trailer
• Verify the change in all affected pipelines.