ACR Observability and Metrics: Expose last image pull timestamp (last image pull date) for purging scenarios

It would be fantastic if the purge functionality would enable purging based on last pull date instead of modified date. This would allow users to purge images that have not been pulled in the last 90 days, as an example.

For advanced container approaches that regularly build/rebuild their environments, the last pulled date is a good indicator of images that have become stale and are no longer needed.