feat: support namespace expiry configmap

[shreddedbacon]

Checklist

• Affected Issues have been mentioned in the Closing issues section
• Documentation has been written/updated
• PR title is ready for changelog and subsystem label(s) applied

Supports the existence of a kind: ConfigMap that contains data["timestamp'] in the same format that the namespace timestamp label would contain. The standard namespace terminator cronjob will check for this and then check the namespace labels.

If the configmap exists, it will prefer this over the namespace label.

Closing issues